The stolen Microsoft certificates it revoked on Monday were used by the Flame malware to launch a man-in-the-middle (MITM) 'spoofing' attack on the Microsoft Update security update system, according to F-Secure.
Microsoft Update is the very system Microsoft employed to distribute Monday's emergency security update. The service is used to issue security updates for Internet Explorer, Office, and Windows Live.
Microsoft revoked trust for two Microsoft Root Authority certificates and one from Microsoft Root Certificate Authority on Sunday after discovering that its Terminal Services licensing certificates--normally issued for enterprise server license verification--could be used to sign any code as having been made by Microsoft.
Microsoft's security update prevents attackers from signing code that fraudulently validates any software as Microsoft's.
According to F-Secure's chief research officer, Mikko Hypponen, the fake certificates were used to validate one of Flame's modules which attempts to do a MITM attack on Microsoft Update. If successful it drops a file called "WUSETUPV.EXE".
"This file is signed by Microsoft with a certificate that is chained up to Microsoft root. Except it isn't signed really by Microsoft," wrote Hypponen in an update Monday evening.
"Having a Microsoft code signing certificate is the Holy Grail of malware writers," said Hypponen.
Malware that spoofs and replicates via Microsoft's update system was the "nightmare scenario", he said. The only upside was it wasn't used in a large financially motivated attack, but rather a small scale targeted attack, he said.
HD Moore, chief technology officer for Rapid 7 explained to CSO.com.au Microsoft certificates are significant because of the high level of trust Windows systems place in its root certificates.
"There's a lot of components in Windows that will only load if they're signed by a root authority. Things like ActiveX controls may not pop up a warning if they're signed by Microsoft. The same thing applies to kernel modules," said Moore.
"If you have a rootkit and you install the kernel module you could either get it signed by Global Sign, another kernel vendor, in which case a big pop up occurs, asking 'Do you want to install this driver by this vendor?'. So even in the best case scenario, you will still get a pop up for those drivers. But if it's signed by the Microsoft Root Authority, it bypasses all those pop ups and often bypasses the AV detections as well because they say, 'hey, this is signed by someone we trust, I'm not going to bother signaturing it."
The stolen certificates in this case would allow an attacker to install "rogue updates" using a Domain Name Service (DNS) MITM.
"The ActiveX control/Windows update system lets anyone with DNS control install updates, but only if the update is signed by Microsoft," explained Moore.
The attack would be "incredibly difficult to detect", however, even with a Microsoft signature the attacker would still need to force the update system to hit the malicious server, said Moore.
"It still doesn't seem that useful for breaking into new systems... but it does make keeping access easy, if you can subvert DNS," said Moore.
While Stuxnet also used a stolen certificate from RealTek, Moore said Flame doesn't appear to show anywhere near the level of complexity of Stuxnet.
"A lot of malware these days ships with stolen certificates. The original Stuxnet shipped with a stolen RealTek key. It seems like stealing a code-signing key hasn't been much of a problem for the folks that are doing targeted attacks.
"The only thing that jumped out as being incredibly difficult (in Stuxnet) were the PLC code--actually modifying the hardware etcetera--and some of the exploits were just off the wall crazy and really good bugs that I'm surprised no one ever found before.
"For Flame, it's not quite as clear that it's anywhere near the same level of complexity or that it really has anything sophisticated. It didn't sound like there were any new vulnerabilities or new exploits being exploited by it, it just was basically a new automation kit."