Wednesday, December 21, 2005

Cross Site Scripting Discovered in Google

"This is reported as a Google.com bug, which is partially true. But this is only one half of the problem. The other half of the problem (mentioned in the full article) is due to a dubious feature in Internet Explorer: when it gets a page without a specified character encoding, it does not rely on default values for the encoding (which should be iso-8859-1 for HTML or UTF-8 for XHTML).Instead, Internet Exploerer tries to guess the encoding of the contents by looking at the first 4096 bytes of the page and checking the non-ASCII characters. In the case of the cross-site scripting attack decribed here, the problem is that IE would silently set the encoding of a page to UTF-7 in case some characters in the first 4096 bytes looked like UTF-7. This silent conversion to UTF-7 by Internet Explorer in a text that Google assumed to use the default encoding allowed the attackers to bypass the way Google was filtering "dangerous" characters in some URLs.The article puts the full blame for the vulnerability on Google.com. I think that a part of the blame should also be shared by the Internet Explorer designers (and any other browser that does unexpected things while trying to guess what the user "really meant")."

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