Facebook Bug Bounty 2014, Reflected XSS and Filter Evasion worth 7500$

6:04 AM

This is the second part of the series on how I found an XSS in Facebook. I wrote about the first
bug here. It was a Linkshim evasion and URL redirection bug. I used “../http://site.com” in the
continue parameter for redirection, bypassing the Linkshim and I explained how there.

After Facebook triaged my bug, promised me a 1000$ bounty and after fixing the issue, I
realized something. The bug could have been a cross-site scripting issue. How? well, I don't
know how the hell I missed this in the first place but when you give Linkshim “../http://site.com”
to sanitize, the parameter renders the following code (first bug)

<a href=”http://site.com”>Continue</a>

You know what that means, if I gave it:

“../data:text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4K#” it will become,

For those of you who can’t do base64 decode in your head, that is equivalent to
“<script>alert(“XSS”);</script>” and I put the hash (#) tag behind to make sure other parameters
following it can be ignored as not a part of the Base64

Or simply by giving the parameter “../javascript:alert(0);//”, that will create a code

<a href=”javascript:alert(0);//”>Continue</a>

The above href attribute is properly sanitized and converted into entities. But since both the functions
htmlentities() and htmlspecailchars() don’t filter the above payload it was possible to execute a
reflective XSS when a user clicked the Continue button. And the final payload would look
something like

https://m.facebook.com/feed_menu/?story_fbid=808015282566492&id=100000740832129&
confirm=h&continue=../javascript:alert(0);&perm&no_fw=1&_rdr

Makes me wonder what I can do with it, stil da qawkies? Lmao.

So simple and yet effective. I reported this after the URL redirection has been fixed (making it
impossible to verify the XSS) but FB security was kind enough to understand the issue this
could’ve made and reconsider the first bounty to 2 type of injections in one parameter
(XSS, Open Redirection / Linkshim Evade) and raise the bounty up to 7500$.