Microsoft issues temp fix for serious Windows security bug

Mitigates script injection threat

Microsoft has warned customers to apply a temporary security fix to protect against a serious, newly discovered security bug in all supported versions of Windows.

The vulnerability results from the way Windows processes webpages containing MIME-formatted content. Attackers can exploit the weakness to run malicious scripts that steal sensitive information, spoof trusted websites or carry out other actions not authorized by the user. Internet Explorer is the only attack vector for the vulnerability, which resides in the Windows implementation of the MHTML protocol.

Microsoft's security team is still studying the flaw and “will take the appropriate action” once the investigation is complete. In the meantime, the company is advising Windows users to install a temporary “fix it” to prevent attacks. The measure disables some legitimate script execution and ActiveX functionality within MHT documents, but these side effects are mostly limited, members of Microsoft's security team blogged.

The security team is working with website operators, including Google, to explore possible server-side fixes as well. Potential fixes include filtering newline characters out of requests and responses, prepending newline characters onto HTTP responses, and altering the status code of HTTP responses.

Of course, a measure that's easier for most Windows users is to use an alternate browser, since IE is the only known vector.

The vulnerability and proof-of-concept code was recently posted here. Microsoft's advisory said there's no evidence the flaw is being actively exploited in the wild. The company has additional information here. ®

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