http://www.theregister.co.uk/2010/04/20/microsoft_ie_xss_fix/ 

By Dan Goodin in San Francisco 
The Register
20th April 2010

Microsoft will release an update intended to rid Internet Explorer 8 of 
a vulnerability that can enable serious security attacks against 
websites that are otherwise safe.

The change, which will be introduced in June, will be the third time in 
six months that Microsoft has tweaked a feature used to filter out XSS, 
or cross-site scripting filter, attacks against websites. The filter, 
which Microsoft introduced with the release of IE 8, is designed to 
strip out malicious commands that exploit the vulnerabilities, which 
plague many websites.

As The Register reported in November, the new XSS filter could be 
exploited to introduce XSS attacks on sites that otherwise weren't 
vulnerable. Microsoft has twice made changes to the feature, once in 
January and again in March, but last week, researchers at the Black Hat 
Security Conference in Barcelona showed the filter still injected 
threats into sites that included Google, Wikipedia, Twitter and even 
Microsoft's own Bing.

"This issue manifests when malicious script can 'break out' from within 
a construct that is already within an existing script block," David 
Ross, of Microsoft Security Response Center, said here. "While the issue 
identified and addressed in MS10-002 was identified to exist on 
high-profile websites, thus far real-world examples of the SCRIPT tag 
neutering attack scenario have been hard to come by."

[...]


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