Microsoft ships rush patch for Windows shortcut bug

As promised, Microsoft today issued an emergency patch for the critical Windows shortcut bug attackers have been exploiting for several weeks.

Also as pledged, Microsoft did not deliver a fix for users running Windows XP Service Pack 2 (SP2) or Windows 2000, which were retired from support three weeks ago.

There was little in Monday's accompanying bulletin that wasn't already known, noted Andrew Storms, director of security operations at nCircle Security.

"This was almost entirely public at this point," said Storms. "The only question I had was whether Microsoft would try and release a patch for unsupported operating systems."

Storms' reference was to XP SP2 and Windows 2000. "There's a ton of people still running SP2, and it just went end-of-life," Storms argued. "And SCADA systems typically run on older versions of the OS. I thought Microsoft might be strong-armed by SCADA vendors into releasing a fix for SP2."

But Microsoft stuck to its long-standing policy and did not provide patches for machines running Windows XP SP2, Windows 2000 or any other off-support version.

The vulnerability addressed today was first described in mid-June by VirusBlokAda, a little-known security firm based in Belarus, but attracted widespread attention only after security blogger Brian Krebs reported on it July 15. A day later, Microsoft admitted that attackers were already exploiting the flaw using the "Stuxnet" worm , which targets Windows PCs that manage large-scale industrial-control systems in manufacturing and utility firms.

Those control systems are often dubbed SCADA, for "supervisory control and data acquisition."

The flaw was in how Windows parsed shortcut files, the small files displayed by icons on the desktop, on the toolbar and in the Start menu that launch applications and documents when clicked. By crafting malicious shortcuts, hackers could automatically execute malware whenever a user viewed the shortcut or the contents of a folder containing the malevolent shortcut.

Exploit code went public last month, and Microsoft and others have spotted several attack campaigns based on the bug. When the company announced last Friday that it would patch the shortcut bug today, it also said that it had seen the virulent "Sality" malware family using the shortcut exploit.

Microsoft patched the problem by "correctly validating the icon reference of a shortcut," according to MS10-046 bulletin.

The company also told users who had deployed a recommended workaround -- which involved disabling the displaying of all shortcuts -- to undo that workaround after applying the patch. Scattered reports on the Web, however, have noted problems unless the workaround is reversed before the patch is applied.

Because Microsoft's patch results in a new version of Shell32.dll being pushed to users, the quality of the update will be important: Shell32.dll is a crucial Windows library file that contains numerous Windows Shell API (application programming interface) functions. If it's flawed, or incorrectly updated on some machines, PCs will lock up with the notorious Blue Screen of Death.

Storms didn't think there was anything to worry about. "They patched a Windows kernel bug in 20 days back in January," Storms pointed out. "They probably understand the risks here by going out-of-band."

Jason Miller, data and security team manager for Shavlik Technologies, said he wasn't expecting a rush patch because of the proximity of August's regular security updates.

"It's not uncommon for Microsoft to release out-of-band," said Miller, "but Patch Tuesday is just a week away. I expected that they would just wait until then."

Microsoft's regularly-scheduled monthly updates are to ship Aug. 10, a week from tomorrow.

According to Miller, out-of-band updates are usually released in-between a pair of Patch Tuesdays, in other words, approximately two weeks before the next slated release.

"Microsoft must have seen something in this that prompted them to release now," said Miller, referring to last Friday's announcement that Sality had begun exploiting the shortcut bug. "I'd bet that they're probably expecting that we'll see an additional uptick in attacks as other viruses add this [exploit] to their payloads," Miller concluded.

The patch, which is available for all still-supported versions of Windows, including XP SP3, Vista, Windows 7 , Server 2003, Server 2008 and Server 2008 R2, can be downloaded and installed via the Microsoft Update and Windows Update services, as well as through Windows Server Update Services.