Microsoft is releasing an off-cycle patch Tuesday for the .ANI vulnerability that saw an escalating number of threats appearing over the weekend.
"From our ongoing monitoring of the situation, we can say that over this weekend attacks against this vulnerability have increased somewhat," Christopher Budd, security program manager at Microsoft's Security Response Center, wrote in a blog Sunday. "Additionally, we are aware of public disclosure of proof-of-concept code. In light of these points, and based on customer feedback, we have been working around the clock to test this update and are currently planning to release the security update that addresses this issue on Tuesday, April 3, 2007."
Budd added that Microsoft's technicians will be testing the patch, which will be released on an 'as is' basis with no warrantees, right up until its release. It's possible, he noted, that they will find an issue that will force the release to be delayed.
The amount of attacks against the vulnerability intensified over the weekend, according to F-Secure, which noted that the first worm using the exploit was discovered roaming the Internet on Sunday. "We've seen a lot of activity relating to the .ANI exploit during the weekend," said Mikko Hypponen, chief research officer at F-Secure, in an e-mail to InformationWeek. "This vulnerability is really tempting for the bad guys. It's easy to modify the exploit, and it can be launched via Web or e-mail fairly easily."
Websense Security Labs reported that researchers there now are monitoring more than 100 Web sites that are spreading the .ANI zero-day exploit. Proof-of-concept code also is in the wild.
"Currently, the majority of the attacks appear to be downloading and installing generic password-stealing code," Websense reported on its blog. "Most sites are hosted in China. Interestingly, the most popular domain space being used is .com."
The .ANI vulnerability lies in the way Windows handles malformed animated cursor files and could enable a hacker to remotely take control of an infected system. The bug affects all the recent Windows releases, including its highly-touted Vista operating system. Internet Explorer is the main attack vector for the exploits.
"In order for this attack to be carried out, a user must either visit a Web site that contains a Web page that is used to exploit the vulnerability, view a specially crafted e-mail message, or open a specially crafted e-mail attachment sent to them by an attacker," Adrian Stone, a Microsoft researcher, said in a blog. "While the attack appears to be targeted and not widespread, we are monitoring the issue and will update the advisory and blog as new information becomes available."
Last Friday, eEye Digital Security released a patch designed to prevent the latest exploit from working.
The Zeroday Emergency Response Team (ZERT) also released a patch "which addresses the core of the vulnerability, by ensuring that no more than 36 bytes of an "anih" chunk will be copied to the stack buffer, thus eliminating all potential exploit paths while maintaining compatibility with well-formatted animated cursor files." The patch is available for Microsoft Windows 98, 2000, XP, Server 2003, and Vista.
The Internet Storm Center is advising users that this is an unofficial patch and should be removed when Microsoft releases its own patch.