Wednesday, December 17, 2008

Firefox cross-domain text theft....

... and a reappearance of the "302 redirect trick".

Here's the second bug from my PacSec presentation, and it's another Firefox one; kudos to the Firefox security team for their responsiveness. It's fixed in the recent and 3.0.5 releases.

It involves, yes, a cross-domain <script src="blah"> tag. These remain a horrible wart in web app security; you have to make sure that any authenticated resource on your domain either does not have any side effects when parsed / executed as JavaScript, or is CSRF protected.

This particular bug involves Firefox's window.onerror handler, which reports on JavaScript parse and execution errors. This handler has previously been used by Jeremiah Grossman to determine login status via script errors, see here! (Whereas this hole can be closed, it's not clear my similar attack via CSS can be).

The new attack notes that certain JavaScript error messages leak real content from remote domains, for certain constructs of data. More in-depth technical detail is here:

One cute twist is that Firefox 3 already had this fixed (thanks to Filipe Almeida; see credit below), but the "302 redirect trick" bypassed that fix. This trick is becoming quite fruitful; see previous Firefox image theft bug.

Credit to Filipe Almeida for being awesome. He was playing with this stuff long before anyone else.

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