CRSF Defense Using Content Injection Support By ModSecurity

The most advanced and imaginative use of the content injection feature is that devised by
Ryan C. Barnett, the ModSecurity Community Manager and author of the Core Rule Set. He
established a way to use content injection to defend vulnerable applications against Cross-
Site Request Forgery (CSRF) attacks, otherwise only possible through the modification of
the source code of the vulnerable applications. (If you are not familiar with CSRF, I suggest
that you read through the CSRF entry on Wikipedia [http://en.wikipedia.org/wiki/Crosssite_
request_forgery].)
The usual way to defend against CSRF is to embed special tokens into application forms, and
accept only those submits that contain the correct token values. CSRF requests faced with such
defenses always fail, because they have no way to “know” the correct token value.
Ryan’s approach was to use content injection to inject JavaScript into all application pages,
which is then used to modify all page forms to add tokens where they wouldn’t normally exist.
In the second part of the trick, he would have ModSecurity rules inspect all POST requests to
verify that they contain the correct values. Brilliant!
For more information, look up Ryan’s Black Hat DC 2009 whitepaper WAF Virtual Patching
Challenge: Securing WebGoat with ModSecurity. The 26-page document contains many other
interesting techniques.