The details of the collision attack used by the Flame malware authors
to create a forged code-signing certificate for Microsoft code are beginning to emerge, and the company said that the attackers used an MD5 hash collision specifically to ensure that their attack would work on machines running Windows Vista and later versions of the OS.
Microsoft also said that it will roll out some hardening changes to its Windows Update infrastructure to prevent the kind of man-in-the-middle attack that Flame used.
Microsoft officials said that as they began to analyze the components of the Flame malware, it noticed that the digital certificate used by the malware not only chained up to the Microsoft root CA, but it had some odd attributes that didn't add up.
"As we reviewed this certificate, we noticed several irregularities. First, it had no X.509 extension fields, which was not consistent with the certificates we issued from the Terminal Server licensing infrastructure. We expected to find a Certificate Revocation List (CRL) Distribution Point (CDP) extension, an Authority Information Access (AIA) extension, and a “Microsoft Hydra” critical extension. All of these were absent," Microsoft's Jonathan Ness