Microsoft Corporation released a security bulletin which revokes three certificates that were used to sign some modules of Flame – a highly sophisticated spy malware. According to Kaspersky Lab's Aleksandr Gostev modules called Gadget and Munch are responsible for spreading Flame throughout local networks.
Most of information security experts used to think that the malware infects its victims via a zero-day vulnerability. This notion was partially confirmed by the fact that even fully patched Windows 7 computers are exposed to the threat. But discovery of two modules that conduct man-in-the-middle attacks against other computers made the zero-day theory almost refuted.
"When a machine tries to connect to Microsoft's Windows Update, it redirects the connection through an infected machine and it sends a fake, malicious Windows Update to the client," the researcher said.
The rogue update contains a number of files with WuSetupV.exe – the Flame dropper, among them. This file is signed by Microsoft certificate and it can be run without any warning and user interaction.
"The interception of the query to the official Windows Update (the man-in-the-middle attack) is done by announcing the infected machine as a proxy for the domain. This is done via WPAD. To get infected, the machines do need however to have their System Proxy settings configured to 'Auto'," - Gostev pointed out.
Experts are still not certain about the way the virus infects the first computer in a network, and still believe that a zero-day vulnerability exploit may exist.
Aleksandr Gostev's advisory is accessible here.
