Microsoft today announced it will issue an update to its Windows Update to prevent copy-cat hackers from duplicating Flame's feat of infecting fully-patched PCs by faking the service.
The company also described in more detail how Flame's authors were able to spoof Windows Update.
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On Sunday, Microsoft acknowledged that Flame -- the super-espionage toolkit that has infected Windows PCs throughout the Middle East, but appears to have been aimed at Iran in particular -- used fraudulent code-signing certificates generated by abusing the company's Terminal Services licensing CA (certificate authority), which is normally used by enterprises to authorize remote desktop services and sessions.
Later, Microsoft also confirmed that those certificates were used to sign bogus updates that were force-fed uninfected PCs by a Flame-compromised computer on the same network.
Researchers at Kaspersky Lab and Symantec used their forensics analyses to more completely describe how Flame managed the feat.
Today, Microsoft said that Flame was able to trick Windows XP machines into accepting the phony Windows Updates once they generated digital certificates with Microsoft's own "signature."
But to dupe Windows Vista and Windows 7 systems, the hackers had to go a step further.
To do that, they leveraged several weaknesses in Microsoft's certificate infrastructure and signing to perform a cryptographic "collision attack," where two different values produce the same cryptographic "hash."
Jonathan Ness, an engineer with the MSRC (Microsoft Security Response Center), explained the results.
"After [the collision] attack, the attacker had a certificate that could be used to sign code that chained up to the Microsoft Root Authority and worked on all versions of Windows [emphasis added]," Ness wrote today on the Security Research & Defense blog.
The combination of the flaws in the Terminal Services' CA and the collision attack made it possible for Flame to hoodwink Windows Vista and Windows 7 PCs as well as those running the 11-year-old XP.
Microsoft's Windows Update team also blogged Wednesday to explain how it plans to better secure Windows' default update mechanism, which is used by hundreds of millions of PCs worldwide, to prevent a repeat of the Flame tactic.
An update for Windows Update will be pushed to users later this week that will force the service to acknowledge only certificates issued from a new authority the company will create, and no longer accept other Microsoft-signed digital signatures, as it has since its inception.
"Second, we are strengthening the communication channel used by Windows Update in a similar way," the blog stated.