full-disclosure - XSS + XSRF/CSRF...

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Date: Thu, 15 Feb 2007 10:32:39 +0100
From: Baey <baey@...pl>
To: full-disclosure@...ts.grok.org.uk
Subject: XSS + XSRF/CSRF...

Recently I've been testing some methods or semi-methods of securing web applications against
XSRF/CSRF attacks (crypto tokens, POST instead of GET, Referer header validation, etc.).
Generally these techniques work (more or less) but there is a major flaw in such thinking:
we start to trust the data that comes from the user's browser only on the basis of that user
has hit the final (tokenized/critical) webpage thru surfing the webapp data flow, not just
hitting it directly (like clicking on some evil link).
This is where XSS come into play. If the same webapp is vulnerable to cross-site scripting
the attacker can '0wn' the user's browser using some fancy tools like XSS Proxy, XSS Shell,
BackWeb (aka Backframe) or BEEF. Or just by using XMLHttpRequest or FlashRequest (he's still
in the context of the valid user's session and the same domain) to totally break the anti-XSRF
techniques.

So how to defend against such powerful XSS+XSRF attack ?
The one what comes into my mind is to use something that only valid user can have (SecureID,
one time pass, etc.) or the user is supposed to authorize the critical action (ie. updating
profile, funds transfer, etc.) by 'hard-to-read' validation images (aka CAPTCHA).
Jeremiah Grossman has pointed out (http://jeremiahgrossman.blogspot.com/2007/01/preventing-csrf-when-vulnerable-to-xss.html)
some interesting method of disabling some XSS functionality in the browser itself by overwriting
constructors of objects or redefining functions. It surely can help but it also dooms all
Ajax-aware apps.
Stefan Esser has also put some 2c into the topic (http://blog.php-security.org/archives/48-CSRF-protections-are-not-doomed-by-XSS.html)
He suggests to embedd all HTML forms in IFRAMEs which source is pointed out into another domain.
That definitively stops XSS, but it can horrify all web developers who in that case have to
rebuild whole web application architecture.

So as I see it, there is no one remedy. Defense in depth once again.

Cheers.
BK
Security Tshooter

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