gorilla/csrf CSRF demo

what

this repository contains a demonstration CSRF attack exploiting a vulnerability in gorilla/csrf based on broken referer checking.

This vulnerability allows an attacker who has gained XSS on a subdomain or top level domain to perform authenticated form submissions against gorilla/csrf protected targets that share the same top level domain.

This bug has existed in gorilla/csrf since its initial release in 2015.

how and why

gorilla/csrf examines the contents of r.URL.Scheme to determine whether a request is being served over HTTPS and only runs the Referer check if this is the case. However, per the Go net/http#Request documentation, these values are never set for "server" requests, and so this check never executes.

   // URL specifies either the URI being requested (for server
   // requests) or the URL to access (for client requests).
   //
   // For server requests, the URL is parsed from the URI
   // supplied on the Request-Line as stored in RequestURI.  For
   // most requests, fields other than Path and RawQuery will be
   // empty. (See RFC 7230, Section 5.3)
   //
   // For client requests, the URL's Host specifies the server to
   // connect to, while the Request's Host field optionally
   // specifies the Host header value to send in the HTTP
   // request.
   URL *url.URL

Even more unfortunate, these values are populated when constructing http.Request objects with http.NewRequest passing full URLs as happens in the library's test suite. This means that the unit tests are in effect "cheating" as they have access to information in the r.URL struct that will never be available to a production application.

instructions to replicate demo

1. install mkcert
2. use mkcert to create a certificate for the *.example.test domain
3. point each of the following domains to localhost in /etc/hosts or similar.

• attack.example.test
• safe.example.test
• target.example.test
• trusted.example.test

4. run sudo go run main.go -tls-cert=YOUR_CERT_FILE -tls-key=YOUR_KEY_FILE -domain=example.test
5. navigate to https://target.example.test & use the forms to make requests to https://attack.example.test and https://safe.example.test