Microsoft admits new ActiveX zero-day bug

As patch day looms, company says critical flaw affects Office users running IE

For the second time in a week, Microsoft is warning users that hackers are exploiting an unpatched, critical bug in a company-made ActiveX control, putting people running Internet Explorer (IE) at risk.

The company has been busy lately acknowledging "zero-day" vulnerabilities. Today's admission was the third in the last two months, and the fifth since February.

According to the security advisory Microsoft released early Monday, the vulnerability is in Office Web Components, a set of ActiveX controls for publishing Office content to the Web, and for displaying that content in IE. The bug is in the ActiveX control that displays Excel spreadsheets within IE, Microsoft said.

The timing of the disclosure was particularly awkward for Microsoft. Top executives spent much of Monday morning touting the new Office 2010, which will release simultaneously next year with Office Web, scaled-down online versions of Word, Excel, PowerPoint and OneNote.

Users running Office XP, Office 2003, Internet Security and Acceleration Server (ISA) 2004, ISA 2006 and Office Small Business Accounting 2006 are at risk from attack through IE, Microsoft said. It classified the bug as a "critical" threat. "This vulnerability could be used for remote code execution in a 'browse and get owned' scenario," said Fermin Serna of the Microsoft Security Response Center (MSRC) in a blog entry today.

Hackers are now exploiting the bug in the wild, Microsoft admitted.

As in its warning of last week, Microsoft again said the most likely attack scenario would involve a malicious Web site hosting the exploit. U.K.-based Sophos echoed that with some specifics today, saying that it has found several sites, "mostly hosted in China that serve the exploit as a part of a Web exploit kit."

People running non-Microsoft browsers, such as Mozilla's Firefox or Google's Chrome, are not at risk, as they don't support ActiveX.

Users with Office 2007 are not vulnerable to attack, at least by default, although they may be if they have manually downloaded and installed Office Web Components 11, the version normally bundled with Office 2003.

"[We're] working to develop a security update," confirmed Dave Forstrom, an MSRC spokesman. He did not offer up a release date, however. "This update will be released once it reaches an appropriate level of quality for broad distribution," Forstrom said in a entry on the MSRC blog.

In lieu of a patch, users can protect themselves by setting two "kill bits" to block Office Web Components from running in IE. Since setting ActiveX kill bits can be dangerous -- it requires editing the Windows registry -- Microsoft has again created an automated tool to do the heavy lifting. The so-called "Fix it" tool can be downloaded from Microsoft's support site.

Microsoft's next regularly-scheduled security updates are due tomorrow, when it expects to roll out a half-dozen bulletins. The company has already promised to push a kill bit update for last week's ActiveX bug, but was unsure Monday whether it would be able to do the same for the newest vulnerability.

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