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Microsoft 'hardens' Windows Update from Flame penetration

How the hot malware burned a new hole in Redmond's backside

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Microsoft has "hardened" its Windows Update system after researchers discovered the Flame virus can infect PCs by offering itself as an update masquerading as official Microsoft software.

The sophisticated worm has been hurtling through computers in the Middle East and beyond for up to two years before being unearthed by security experts late last month. Now it has emerged that the malware uses a skeleton-key-like certificate found in Microsoft's Terminal Services Licensing server to sign its malicious code and trick Windows machines into trusting and installing its executables.

Redmond said in a blog post yesterday that it was continuing to analyse Flame and repeated that it would "evaluate additional hardening of both the Windows Update channel and our code signing certificate controls".

It warned any customers who do no have their Windows Update software set to automatic configuration to install the latest patch immediately, which will thwart Flame's man-in-the-middle attack. Microsoft explained:

To attack systems using Windows Vista and above, a potential attacker would have needed access to the now invalid Terminal Server Licensing Service certificates and the ability to perform a sophisticated MD5 hash collision.

On systems that pre-date Vista, an attack is possible without an MD5 hash collision. In either case, of course, an attacker must get his signed code onto the target system. This can be done if the client’s Automatic Update program receives the attacker’s signed package because such packages are trusted so long as they are signed with a Microsoft certificate.

Windows Update can only be spoofed with an unauthorised certificate combined with a man-in-the-middle attack. To address this issue, we are also taking steps to harden the Windows Update infrastructure and ensure additional protections are in place.

Microsoft added that it had waited until it was clear that most of its customers were protected against the malware before publishing more details about how so-called "cryptographic collisions" had been used in those attacks.

Earlier this week, virus protection vendor Symantec described exactly how Flame, AKA Flamer, could spread on local networks.

The security company noted: "One of the methods is to hijack clients performing Windows Update. Three Flamer apps are involved in delivering the rogue update: SNACK, MUNCH, and GADGET."

SNACK tended to sniff out NetBIOS requests on LANs and would then imitate a Web Proxy Auto-Discovery Protocol (WPAD) server and feed a rogue configuration file (wpad.dat) to the local network, thereby effectively hijacking it and forcing traffic to redirect to the malware-infected machine, Symantec said.

MUNCH - a web server within the Flame code - would then chow down on the redirected traffic, including matching URLs for Microsoft's Windows Update software.

The final part of the puzzle was GADGET, a module which Symantec said provided a binary signed by the dodgy Terminal Services certificate via the MUNCH web server that fooled the system into believing that it was the genuine article from Microsoft.

"The binary is downloaded by the uninfected computer as if it is a legitimate Windows Update file and is executed. The binary is not Flamer itself, but a loader for Flamer. One sample of this binary refers to itself as TumblerEXE.exe," Symantec explained.

Hence, all the panic coming out of Redmond towers to ensure that its customers have all updated their Windows software to prevent their systems being compromised by Flame. ®

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