Microsoft has upgraded up a key defensive feature of 64-bit Windows Vista to help protect the operating system against hacks that have plagued it for weeks.

The update to Vista's Kernel Patch Protection - PatchGuard - was issued through Windows Update as a high-priority download, but not as a patch per se. Microsoft, in fact, denied that it was a security fix. "While this update adds additional checks to the Kernel Patch Protection system, it does not involve a security vulnerability," an advisory posted by the Microsoft Security Response Center (MSRC) stated. "The update does increase the reliability, performance, and resiliency provided by Kernel Patch Protection."

Although the update targets all 64-bit editions of Windows, it's Vista that stands out by reason of recent events. Since late July, a pair of utilities have sidestepped a crucial Vista security feature that requires drivers to be signed by a valid digital certificate. Both utilities piggybacked unsigned code onto a legitimate driver to get the former past Vista's defences and into the kernel.

First off the mark four weeks ago was Australian developer Linchpin Labs, which released Atsiv (Vista spelled backward), a utility that allowed users to load unsigned drivers to the Vista kernel. Within days, Microsoft had the certificate revoked, forcing Linchpin to throw in the towel.

Next, Canadian researcher Alex Ionescu last week took advantage of a flaw in a Vista video driver from AMD's ATI Technologies unit to unveil Purple Pill, another utility that allowed unsigned drivers to be loaded into the kernel. Ionescu quickly pulled Purple Pill once he realised that the ATI driver had not been patched.

"[Purple Pill] had embedded in it an ATI signed driver that would be dropped to disk and loaded (a similar approach to Atsiv)," said Symantec analyst Ollie Whitehouse in a posting to the company's security blog last week. "However it would appear that this signed driver contained a design error which allows you to use it to load any arbitrary driver even if they are not signed."

For its part, ATI refreshed its Catalyst video driver for Vista on Monday to patch against a repeat of Purple Pill, fulfilling a promise made earlier by AMD in a statement posted by ZDNet blogger Ryan Naraine.

While Catalyst 7.8 may have plugged the hole in ATI's driver, more driver vulnerabilities or design flaws would likely be found, or others would take the Atsiv approach and pay the money for a certificate. "Let's hope Microsoft steps in and uses Windows Update as an upgrade mechanism for them," Whitehouse said in a post.

But that's not what appears to have taken place on Tuesday as Microsoft updated PatchGuard, he added in an email exchange today.

"There is very little, if anything, Microsoft can do to stop the piggybacking [of drivers] if someone is willing to go to the effort of obtaining a signing certificate for their own driver," said Whitehouse. "The only real thing Microsoft could do to improve this process would be... to start performing code reviews of all drivers wishing to be signed. But in reality it's not scalable. Even then, it would become a game of cat and mouse with regards to individuals determined to get code through the review process."

Instead, Whitehouse went on, what Microsoft seems to have done is harden PatchGuard's defences so that when a piggyback attack does take place - for instance, a hacker uses a legitimate driver to inject his own code into the Vista kernel - the damage is minimised.

"It looks like they are trying to make it harder to do anything malicious once you've exploited vulnerabilities which allow code to be executed in the kernel, such as ATI driver/Atsiv, and so on," he said.

Microsoft wasn't much help in figuring out exactly what was beefed up by the PatchGuard update; the accompanying information was extremely vague. The MSRC's release manager, Simon Conant, was just as tight-lipped in a posting to the Center's blog. "The update adds additional checks to Kernel Patch Protection for increased reliability, performance and security," Conant said.

Vague or not, Whitehouse applauded Microsoft's move but cautioned against thinking the issue was dead and buried. "While these efforts should be commended, someone simply has to perform sufficient reverse engineering of the Vista kernel in order to locate the PatchGuard functionality in order to target that," he said.

Microsoft and Ionescu, the author of Purple Pill, could not be reached for comment.