A couple months ago I had to deal with an application that was checking the Referer as a CSRF prevention mechanism, but when this header was stripped from the request, the CSRF PoC worked. BTW it is common practice to accept empty Referer, mainly to avoid breaking functionality.
Solutions for Referer header strip
- learn something very cool;
- have a serious headache from all the new info at the end.
This blog post from him is a bit lighter and covers some useful theoretical background, so make sure you read that first before you continue reading this post. He shows a few nice tricks to strip the Referer, but I was wondering; maybe there is an easier way?
Rich Lundeen (aka WebstersProdigy) made an excellent blog post on stripping the Referer header (again, make sure you read that one first before you continue). The HTTPS to HTTP trick is probably the most well-known one, general and easy enough, but it quickly fails the moment you have an application that only runs over HTTPS (this was my case).
The data method is not browser independent but the about:blank trick works well for some simple requests. Unfortunately, in my case the request I had to attack with CSRF was too complex and I wanted to use XMLHttpRequest. He mentions that in theory, there is anonymous flag for CORS, but he could not get it work. I also tried it, but… it did not work for me either.
Krzysztof Kotowicz also wrote a blog post on Referer strip, coming to similar conclusions as Rich Lundeen, mostly using the data method.
Finally, I bumped into Johannes Ullrich’s ISC diary on Referer header and that led to me W3C’s Referrer Policy. So just to make a dumb little PoC and show that relying on Referer is a not a good idea, you can simply use the “referrer” meta tag (yes, that is two “r”-s there).
The PoC would look something like this:
<meta name="referrer" content="never">
<form action="https://vistimsite.com/function" method="POST">
<input type="hidden" name="param1" value="1" />
<input type="hidden" name="param2" value="2" />
*** This is a Security Bloggers Network syndicated blog from Jump ESP, jump! authored by Dávid Szili. Read the original post at: http://jumpespjump.blogspot.com/2016/06/csrf-referer-header-strip.html