Politically motivated attackers are exploiting an unpatched flaw in all supported versions of Microsoft Windows to carry out highly targeted attacks against activists using Google, the company's security team warned.

The unidentified attackers are wielding a serious vulnerability in the way Windows parses webpages containing MIME-formatted content. In January, Microsoft issued a temporary fix for the flaw, which allows attackers to steal sensitive information, spoof trusted websites or carry out other actions not authorized by the user. The company hasn't said when the MHTML bug will be patched.

In a blog post published on Friday afternoon, members of Google's security team said the attacks are also being carried out against users of another popular social website, which they did not identify. Facebook representatives didn't immediately respond to an email seeking comment.

“We’ve noticed some highly targeted and apparently politically motivated attacks against our users,” they wrote. “We believe activists may have been a specific target.”

They reiterated Microsoft's advice that users install the temporary fixit as soon as possible. A better course of action is to halt all usage of the Internet Explorer browser, since that is the sole vector for exploits. This may not be possible for users with custom applications that rely on the Microsoft browser, but even avoiding it while on social networking sites is a good idea, if at all possible.

In addition to issuing the temporary fix, Microsoft has been working with Google to develop server-side protections that mitigate the threat. While those protections are no in place, they shouldn't be relied on.

“These are not tenable long-term solutions, and we can’t guarantee them to be 100% reliable or comprehensive,” Google's blog post warns. “We’re working with Microsoft to develop a comprehensive solution for this issue.”

It's not the first time Google has blown the whistle on what it believed to be political attacks against its users. In January 2010, the company said it uncovered a “highly sophisticated and targeted attack” used to steal Gmail from Chinese dissidents. The so-called Operation Aurora attacks targeted dozens of other companies.

Microsoft updated an advisory published in January to say it “is aware of public proof-of-concept code being used in limited, targeted attacks” and repeated its advice to install the fixit. Vulnerability details and proof-of-concept code were first published here. Microsoft has more about the flaw here. ®