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RE: A technique to mitigate cookie-stealing XSS attacks
From: "Steven M. Christey" <coley () linus mitre org>
Date: Wed, 13 Nov 2002 18:10:47 -0500 (EST)


While this thread has been focused on scripting languages and cookie
theft, that's not the only issue to be concerned about with XSS.

Being able to place arbitrary HTML into an intermediate web page is
dangerous for other reasons (this is sometimes called "HTML
injection," but I view it as another flavor of XSS).  For example,
this would allow attackers to use META-REFRESH style attacks to
redirect victims away from the intended web site.  META-REFRESH has
also been an important component in some vulnerabilities.

More importantly, HTML injection could be used to "anonymously"
exploit certain web client vulnerabilities by inserting malicious HTML
into XSS-vulnerable sites (the only "trace" might be the web site's
logs).  Such attacks do not require cookies and could be
indistinguishable from genuine content.  Not all web client
vulnerabilities require scripting languages to exploit.

As another example, the "FRAME SECURITY=RESTRICTED" feature described
by Michael Howard could be defeated by HTML injection - the attacker
could inject a </FRAME> tag and follow it with the malicious code.
This could also apply to the <dead> tag proposed by Seth Arnold, at
least in some cases.  Despite this, these mechanisms would definitely
have some benefit - especially when a program *tries* to avoid XSS but
doesn't catch every single instance.

Some people argue that XSS is not always serious because attackers
have to be "enticed" to click on links.  But what if, say, a major
bulletin board with thousands of visitors has an XSS bug that allows
an attacker to exploit web client vulnerabilities?

Using XSS style attacks to inject arbitrary HTML into popular web
pages is therefore a serious concern with other implications beyond
what's already been said.  It would be nice to develop protection
mechanisms that address more than just cookie theft, but it seems
difficult when a web document can have so many different content types
scattered in arbitrary locations throughout the document.

- Steve


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