Microsoft on Tuesday warned that attackers have begun exploiting a critical vulnerability in Internet Explorer and rolled out a temporary fix until a permanent patch is issued.
The vulnerability in IE versions 6, 7 and 8, which involves the way the browser handles cascading style sheets, allows adversaries to perform drive-by malware attacks by luring victims to booby-trapped webpages. The exploits are triggered by recursive CSS pages, in which style sheets include their own addresses.
Microsoft confirmed the security flaw in late December. On Tuesday, it updated its advisory to reflect “reports of limited attacks attempting to exploit a vulnerability in all supported versions of Internet Explorer.”
Redmond also issued a workaround that large organizations can implement to protect themselves until a patch is released. It comes in the form of a Fix it that causes IE to reject CSS pages that contain the same URL as a style sheet that's trying to load it.
“This change causes Internet Explorer to refuse to import a CSS style sheet if it has the same URL as the CSS style sheet from which it is being loaded,” Microsoft Security Response Center's Keven Brown explained here. “Simply put, the workaround inserts a check to see if a style sheet is about to be loaded recursively, and if it so, it aborts the load of the style sheet.”
For the workaround to be effective, all existing security updates, particularly MS10-090 released on December 14, must be installed. The temporary fix causes a minor performance decrease – adding about 150 milliseconds to the browsers' startup time – so it should be uninstalled once a patch is put in place. Third-party apps that work with IE should be thoroughly tested before putting the workaround into effect.
The workaround came on the first Patch Tuesday of 2011. As part of the regularly scheduled update release, Microsoft also issued two updates, one in the Windows Backup Manager and the other in Microsoft Data Access Components.
While it was one of the smallest Patch Tuesdays ever, it failed to address at least known vulnerabilities that put Microsoft users at risk. One of them allows attackers to remotely execute malicious code on machines running the XP, Server 2003, Vista, and Server 2008 versions of Windows. Exploit code for it is publicly available. The other, disclosed by Google researcher Michal Zalewski, leads to what he said was a “clearly exploitable crash.” ®