Gumblar is back with a vengeance

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.”

Gumblar is unique as the attackers are gaining access via stolen FTP credentials rather than using the usual code injection methods. In an evolutionary departure from the norm, Gumblar is installing PHP backdoors on the compromised websites and is using the backdoored websites as the actual malware host.

“The implications of this are rather staggering,” adds Landesman. “When a typical outbreak of website compromises occur, there are generally only a few actual malware domains involved. In the case of Gumblar, conservatively there are at least 2,000 backdoored websites serving as actual malware hosts. As a result, there is no single or few points at which to target efforts to shutdown the source of malware.”

To load the malware from the backdoored websites, tens of thousands of other compromised websites have had malicious iframes embedded. Alarmingly, Web surfers who visit one of these conduit sites will be exposed to a collection of exploits designed to silently install the Gumblar malware.

On Windows systems, the installed malware loads when sound-enabled sites or devices are accessed. It also injects itself into the Internet Explorer process and intercepts all Web traffic to and from the computer. Any captured FTP credentials are sent to the attacker thus furthering the growth of the Gumblar website botnet.

Interestingly, an examination of the source code for the malware delivered via the Gumblar-backdoored websites reveals the malware is language-specific, targeting English, Dutch, German, Italian and Spanish users.