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Microsoft prepares post-Flame Windows Update changes

Microsoft heeded calls to feed updates to stymie Flame-like attacks till after Patch Tuesday

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Microsoft will start feeding users an update to the critical Windows Update service in the next few days, several security experts said today.

Windows Update (WU) provides security patches and other fixes to Windows PCs. The service is accessed directly by consumers, and through the Windows Server Update Services (WSUS) component by enterprises.

The update was triggered by the discovery that Flame, a sophisticated, nation state-grade cyber espionage tool, had subverted WU to infect additional PCs within an already-penetrated network. The team behind Flame, which shared code with the makers of the even-better-known Stuxnet worm that sabotaged Iran's nuclear program, pulled off that first-of-its-kind hack by stealing digital certificates from Microsoft.

A week ago, Microsoft announced it would issue an update to WU to prevent copy-cats from duplicating Flame's feat. At the time, it said it would begin serving that update before the end of the week.

Microsoft did, in fact, push the update to some users last week, although it limited the scope of that audience, said researchers.

"It's done and tested, and as we understand it, has been offered to some users," said Wolfgang Kandek, chief technology officer at Qualys, in an interview.

Jason Miller, manager of research and development at VMware, said that he had heard from users who had received the new Windows Update client, and like Kandek, said Microsoft would unthrottle the update - in other words, begin pushing it to all, or at least more, users - "in a few days."

Wait till after Patch Tuesday's 26 flaw fixes

Microsoft also heeded calls to wait until after yesterday's Patch Tuesday to refresh WU by pausing the update, limited though it was, until users' PCs began downloading fixes for the 26 flaws the company delivered this month.

Several researchers, including Kandek and Andrew Storms, director of security operations at nCircle Security, said they had emailed contacts at Microsoft urging the company to wait.

"They released the WSUS update Friday, and started the WU update, but not everyone got it," said Kandek. "Then they put a pause on WU."

Last week, Storms had hoped Microsoft would do the smart thing, and delay the WU update until after Patch Tuesday, noting that to do different might delay some businesses deployment of the fixes.

"They'll want to test the Windows Update update," said Storms last Thursday. "Because if that breaks, everything breaks with it."

The WU update will force the service to acknowledge only certificates issued from a new certificate authority (CA) the company will create, and no longer accept other Microsoft-signed digital signatures, as it has since its inception.

Flame's makers exploited a flaw in Microsoft's Terminal Services licensing CA to generate the fake Microsoft digital signatures. They launched a super-advanced cryptographic "collision attack," where two different values produce the same cryptographic "hash, to gain the bogus certificates.

Have you disabled WU? Might want to turn it back on

Some researchers have argued that the collision attack shows that the Flame team included world-class cryptographers, and would have required considerable computing horsepower to pull off.

"The new WU will be more critical about the certificates that it uses to sign downloads, and be more picky about how it communicates with Microsoft," said Kandek. "It will make the download process more robust."

By the time next month's Patch Tuesday rolls around on 10 July, all users will have had multiple opportunities to grab the WU update, Kandek added. Microsoft will probably make the switch to the new certificates at that time.

Because updates to WU don't rely on users having set the service to automatically receive and install all fixes, everyone who runs WU will receive the update. Windows Update updates are installed whenever the service is engaged, whether automatically, manually or the in-between mode that only notifies users of impending updates.

Only PC owners who have disabled the service and never use it - experts suspect that users running counterfeit copies of Windows avoid it because they fear being found out by Microsoft's sniffing for legitimate licenses when it deploys a new WU client - will not be migrated to the new, more restrictive certificate model.


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