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Martin Paul Eve

Professor of Literature, Technology and Publishing at Birkbeck, University of London

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Following on from a post on sla.ckers it emerges that Firefox has a vulnerability/bug that is very difficult to filter against and allows a fragmented XSS attack.

This is best illustrated by the following example:

<html xmlns="">
		<!-- This is the first injection point: -- -->
		<a href="This is the second injection point: --evadefilter>
		<b style=-moz-binding:url('') />
		<a href=test ">link</a>

The conditions for the XSS working are 2 injection points. Injection point 1 must be inside an HTML comment whilst injection point 2 is inside a double quoted attributed. Here is the above markup replaced to illustrate this:

<!-- This is the first injection point: HERE -->
<a href="This is the second injection point: HERE">

If the first injection point is given as "--" (no quotes) then a nested comment is begun. Injection point 2 should contain --> or --ANYTHING> (which is rendered as a valid comment closing tag in Firefox) to close the comment. At this point the input is being written directly into the document rather than inside the attribute - and all without using the " character which is likely to be filtered.

Thanks to tx and thornmaker at sla.ckers for work on this!