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Microsoft to fix IE8 cross-site scripting bug

Patch to tackle flaw exposed at Black Hat event

Microsoft plans to update Internet Explorer 8 (IE8) in June to stymie attacks that could turn the browser's cross-site scripting filter against websites.

Microsoft's move was prompted by a presentation last week at Black Hat Europe, where researchers Eduardo Vela Nava and David Lindsay showed how IE8's cross-site scripting filter - an anti-malware feature that debuted in a beta of the browser last year - could be used by hackers to launch attacks against sites that would normally be immune. Among the sites that could be abused: Microsoft's own Bing search engine, Digg, Google, Twitter, Wikipedia and "many many more," they said.

IE8 uses what Vela Nava and Lindsay called a "neutering" technique to quash attempted cross-site scripting attacks. The problem is that attackers can manipulate the mechanism for their own purposes. "An attacker may exploit this behavior in order to prevent client-side security functionality from working," said the pair in a paper they published along with their Black Hat presentation (download PDF). "[And] in certain cases [this] can lead to XSS that wouldn't otherwise be possible."

Although Microsoft has dealt with some of the attack scenarios spelled out by Vela Nava and Lindsay in a pair of earlier IE updates - the January and March emergency updates MS10-002 and MS10-018 - yesterday the company said it would issue a cross-site scripting filter update to block another possible attack.

"This change will address a SCRIPT tag attack scenario described in the BlackHat EU presentation," said David Ross, an engineer with the Microsoft Security Response Center (MSRC), in an entry on the group's blog. "This issue manifests when malicious script can 'break out' from within a construct that is already within an existing script block."

Unlike security patches, IE8's cross-site scripting filters are typically updated on-the-fly and in the background, but Microsoft's scheduled this fix for June, rather than immediately, to give the company time for testing, a spokeswoman said today.

Other browsers, including Google's Chrome, also offer cross-site scripting filtering. But according to Lindsay, Chrome users are not at risk to the same kind of abuse.

"Chrome's neutering technique is to completely block [the] page," said Lindsay. "This is preferred over modifying [the] response" as did Microsoft's browser. "IE8 header now allows the same."

Coincidentally, Google patched seven security vulnerabilities in the 'stable' Windows version of Chrome earlier today, including two related to cross-site scripting.

See also:

Internet Explorer 8 review

PC security advice

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