Microsoft issues security advisory on IE exploit, patch in works

New workarounds deal with zero-day exploit aimed at IE

Microsoft Monday night issued a security advisory that provides customers with guidance and workarounds for dealing with a zero-day exploit aimed at Internet Explorer and said a patch is forthcoming.

Earlier in the day, the company said it was investigating the incident which emerged over the weekend when someone published the exploit code to the Bugtraq mailing list. By Monday night, Microsoft switched gears and issued the advisory. There have not been any active exploits reported so far.

Microsoft's Security Response Center posted a blog entry last night saying it was working on a patch. "Our teams are currently working to develop an update and we will take appropriate action to protect customers when the update has met the quality bar for broad distribution. That may include releasing the update out of band."

Microsoft released Security Advisory 977981, which includes workarounds for an issue that exposes a flaw in Cascading Style Sheets that could allow for remote code execution. Vulnerabilities that allow remote code execution generally result in patches rated as critical by Microsoft.

The advisory confirmed the vulnerability affects IE 6 on Windows 2000 Service Pack 4, and IE 6 and IE 7 on supported editions of XP, Vista, Windows Server 2003 and Windows Server 2008. Microsoft said users running IE 7 on Vista can configure the browser to run in Protected Mode to limit the impact of the vulnerability. It also recommended setting the Internet zone security setting to "High" to protect against the exploit. The "High" setting will disable JavaScript, which currently is the only confirmed attack mode.

Microsoft said IE 5.01 Service Pack 4 and IE 8 on all supported versions of Windows are not affected.

Microsoft activated its Software Security Incident Response Process (SSIRP) and said the investigation into the vulnerability is ongoing. SSIRP is a four-step process Microsoft has developed to deal with malicious threats.

Issuing the security advisory is Step 3, called assess and stabilize, where "the engineering team investigates and develops the solution, while the communications team reaches out to provide guidance to customers and partners."

Step 4 is the resolution stage where "the Microsoft Security Response Center provides tools and solutions." While Microsoft did not say specifically that it planned to issue a patch, it did say in an e-mail to media that "the company recommends customers review and implement the workarounds outlined in the advisory until a comprehensive security update is released."

Microsoft also is recommending users upgrade their earlier versions of IE to more recent versions that are not susceptible to the attack, which can give a hacker control of the targeted machine.

Earlier in the day, security experts at Symantec's Security Response division said the published IE exploit code does not work reliably but that a better written version is likely on the way.

"The exploit code is not very good," said Ben Greenbaum, senior research manager with Symantec Security Response. "So it is going to have to be fine tuned before it is a real threat. Right now, it is a potential threat. But it is just a matter of time before somebody finds a far more reliable method for exploiting this."

He says if users disable JavaScript in IE they would be protected against the exploit, however, the action also would break some functionality on Web sites.

Greenbaum did note that JavaScript may not be the only attack vector, but right now it is the only one that has been disclosed publicly.

He says Symantec already has various protections out that would foil an attack by this exploit, and that others are also in the works.

For the attack to be carried out a user only has to be directed to a malicious Web page or visit a legitimate Web page that has been compromised with the exploit code.

IE has become a popular attack target for hackers. Just last month, Microsoft issued a patch rated critical to close a vulnerability in IE first disclosed at the Black Hat conference in July. In addition to IE, Firefox also can be vulnerable to the exploit when it is running the Windows Presentation Foundation plug-in, which gets installed via .Net Framework Service Pack 1.

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