Security researchers have released new details showing that the Flame malware abused Microsoft's Windows Update mechanism to infect other computers through a man-in-the-middle attack.
"When a machine tries to connect to Microsofts Windows Update, it redirects the connection through an infected machine and it sends a fake, malicious Windows Update to the client," blogged Alexander Gostev, head of the Global Research and Analysis team at Kaspersky Lab.
"When a victim updates [a computer] via Windows Update, the query is intercepted and the fake update is pushed," he explained. "The fake update proceeds to download the main body and infect the computer."
According to Symantec's Security Response team, a Flame module called 'Snack' sniffs NetBIOS requests on the local network. NetBIOS name resolution allows computers to find each other on a local network via peer-to-peer communications, opening up an avenue for spoofing.
"When clients attempt to resolve a computer name on the network and in particular make WPAD (Web Proxy Auto-Discovery Protocol) requests, Flamer (Flame) will claim it is the WPAD server and provide a rogue WPAD configuration file (wpad.dat)," Symantec noted. "NetBIOS WPAD hijacking is a well-known technique and many publicly available hack tools have implemented the technique."
"Once a computer that has not yet been compromised receives the rogue wpad.dat file, it will set its proxy server to the Flamer-compromised computer," the firm noted. "All its Web traffic will now be redirected to the Flamer compromised computer first."
According to Symantec, Flame's Munch component serves as a Web server within Flame and receives the redirected traffic. It also checks for a variety of queries, including matching URLs for Windows Update.
"Hijacking Windows Update is not trivial because updates must be signed by Microsoft," Symantec's team added. "However, Flamer (Flame) bypasses this restriction by using a certificate that chains to the Microsoft Root Authority and improperly allows code signing. So when a Windows Update request is received, the GADGET module through MUNCH provides a binary signed by a certificate that appears to belong to Microsoft."
The findings have prompted Microsoft to say that it plans to harden Windows Update against attacks in the future, though the company did not immediately reveal details as to how. On Sunday, Microsoft released an update to revoke the trust it placed in the Microsoft Enforced Licensing Intermediate PCA and Microsoft Enforced Licensing Registration Authority CA signing certificates after components of Flame were found to have been signed with unauthorized certificates.
"The Flame malware used a cryptographic collision attack in combination with the terminal server licensing service certificates to sign code as if it came from Microsoft," blogged Mike Reavey, senior director of Microsoft Security Response Center. "However, code-signing without performing a collision is also possible. This is an avenue for compromise that may be used by additional attackers on customers not originally the focus of the Flame malware. In all cases, Windows Update can only be spoofed with an unauthorized certificate combined with a man-in-the-middle attack."
"To increase protection for customers, the next action of our mitigation strategy is to further harden Windows Update as a defense-in-depth precaution," he added. "We will begin this update following broad adoption of Security Advisory 2718704 in order not to interfere with that updates worldwide deployment."