ScanSafe reported that 29% of all Web malware blocks in October 2009 were the result of Gumblar. This series of website compromises, collectively dubbed “Gumblar” takes a multi-pronged approach, installing traffic sniffers and backdoors on Web surfers’ PCs and then using stolen FTP credentials to compromise and backdoor websites.

In October 2009 Gumblar began leveraging its botnet of backdoored websites uncommonly using them as the malware host itself. The malware hosted on the sites is dynamically constructed at the time of access. Thus different users, dependent on their browser type and other considerations, will be delivered different exploits and potentially different malware.

Worryingly, the malware is also dynamically obfuscated, hampering detection via traditional signature strings.

“Gumblar is arguably one of the most insidious threats facing both Web surfers and website operators today,” comments Mary Landesman, senior security researcher at ScanSafe. “Disturbingly, in early November, we detected that the backdoor left in place on the compromised websites by the Gumblar attackers was being leveraged by other groups of attackers meaning that the sites were under their control. This exacerbates the seriousness of the situation.”

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