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Windows Faces Zero Day MHTML Vulnerability

Microsoft releases temporary fix for bug that allows attackers to run malicious scripts on a user's computer via Internet Explorer.

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Microsoft released a security advisory warning on Friday that Windows is susceptible to exploits that target a bug in the MHTML protocol. Attackers could use the vulnerability to execute client-side scripts that spoof content or steal data.

"The vulnerability exists due to the way MHTML interprets MIME-formatted requests for content blocks within a document," Microsoft said. All supported versions of Windows are affected, except for Windows Server 2008 when installed using the Server Core option.

The vulnerability was first detailed 10 days ago on the WooYun.org Web site, and proof-of-concept code for exploiting the bug has been released. Microsoft, however, said it has yet to see any active attacks that use the exploit.

Interestingly, although the related exploit uses Internet Explorer to launch the attack, the version of IE is irrelevant, as the bug itself is in Windows.

Microsoft described the vulnerability as being similar to a server-side cross-site scripting (XSS) bug, in that attackers can inject malicious code into Web pages and bypass many types of security controls. Such bugs arise when Web applications serve pages containing content that hasn't been properly validated or "escaped," which means stripping out any special characters that an attacker could use to create an XSS attack. According to the Open Web Application Security Project (OWASP), XSS flaws are the most common type of Web application security vulnerability.

While Microsoft has yet to announce when it will release a full fix, it has detailed multiple client-side workarounds, including a registry tweak. On that front, it released a free fix-it tool, aimed at consumers or people who manage a small number of PCs, to automatically apply the suggested registry tweaks.

For larger environments, "I highly recommend you consider deploying the mitigation settings using Group Policy Objects (GPOs) as soon as possible," said Chester Wisniewski, a senior security advisor at Sophos Canada. "It will likely be some time before Microsoft is able to release a patch for this vulnerability and this is one of the cases where it is likely worth the effort to implement the mitigations."

Meanwhile, Microsoft is also working on a server-side fix with a number of Internet service providers, including Google.

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