Temporary Fix for the WMF Exploit

Since Microsoft has decided to wait until Tuesday to release it’s patch for the latest Windows exploit, the WMF security flaw, F-Secure has posted on their site about a fix released by the author of Interactive Disassembler and probably one of the best low level Windows experts in the world, Ilfak Guilfanov. The fix is here.

Ilfak Guilfanov has published a temporary fix which does not remove any functionality from the system (all pictures and thumbnails continue to work normally).

The fix works by injecting itself to all processes loading USER32.DLL. It patches the Escape() function in GDI32.DLL, revoking WMF’s SETABORT escape sequence that is the root of the problem.

This flaw has already spawned dozens of attacks from a MSN Messenger worm to spam that tries to get users to click on malicious web sites.

The vulnerability can be easily exploited in Windows XP with Service Pack 1 and 2, as well as Windows Server 2003, security experts said. Older versions of the operating system, including Windows 2000 and Windows ME, are also at risk, though in those cases the flaw is more difficult to exploit, said Mikko Hypponen, chief research officer at F-Secure.

“We have seen dozens of different attacks using this vulnerability since Dec. 27,” Hypponen said. “One exploits image files and tries to get users to click on them; another is an MSN Messenger worm that will send the worm to people on your buddy list, and we have seen several spam attacks.”

He added that some of the spam attacks have been targeted to select groups, such as one that purports to come from the U.S. Department of State. The malicious e-mail tries to lure the user to open a map attachment and will then download a Trojan horse. The exploit will open a backdoor on the user’s system and allow sensitive files to be viewed.

A chief researcher at F-Secure said,

“We are still far away from a massive virus,” he said. “Most people get attacked by this if they (search for something on the Internet) and get a million results. They may click on a link that goes to a malicious Web site or one that has been hacked, and then get infected.”

In an article from News.com posted today, an antivirus specialist stated that over a million pc’s have been compromised,

More than a million PCs have already been compromised, said Andreas Marx, an antivirus software specialist at the University of Magdeburg in Germany. He has found a hidden Web site that shows how many copies of a program that installs malicious software have been delivered to vulnerable PCs.

“I’m sure it’s just a matter of days until the first (self-propagating) WMF worm will appear,” he said. “A patch is urgently needed.”

So, with Microsoft waiting until Tuesday, attackers are going to have about a week with no worries to try to take advantage of this. So far, most of the attacks have involved installing spyware and adware to display pop up advertising on the infected pc’s.

Microsoft has completed a fix for the problem and is currently testing and localizing the update into 23 languages, the software maker said in its advisory, updated on Tuesday. “Microsoft’s goal is to release the update on Tuesday, Jan. 10, 2006, as part of its monthly release of security bulletins,” the company said.

To protect Windows users, Microsoft shouldn’t wait, but release the patch now, several critics said.

“The flaw is actively exploited on multiple sites, and antivirus provides only limited protection,” said Johannes Ullrich, the chief research officer at the SANS Institute. “Active use of an exploit without sufficient mitigating measures should warrant the early release of a patch, even a preliminary, not fully tested patch.”

Once again, we see a large company not really caring about the users and all they are doing is creating even more ill will.

Added: One of the F-Secure researches stated that one of their test machines became infected after downloading an infected file using the Wget command line tool, without even executing it.

It seems that Google Desktop creates an index of the metadata of all images too, and it issues an API call to the vulnerable Windows component SHIMGVW.DLL to extract this info. This is enough to invoke the exploit and infect the machine. This all happens in realtime as Google Desktop contains a file system filter and will index new files in realtime.