According to ScanSafe, when a typical outbreak of website compromises occur, there are generally only a few actual malware domains involved.
In the case of Gumblar, however, conservatively there are at least 2000 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 the Gumblar malware.
ScanSafe said that internet surfers who visit one of these compromised sites will be exposed to a collection of exploits designed to silently install the Gumblar malware.
Mary Landesman, the ScanSafe's senior security researcher, said that ScanSafe discovered that in October 2009, Gumblar began leveraging its botnet of backdoored websites uncommonly using them as the malware host itself.
The Gumblar malware hosted on the sites is dynamically constructed at the time of access.
In this way, she said, different users - dependent on their browser type and other considerations - will be delivered different exploits and potentially different Gumblar malware.
And, perhaps more worryingly, the Gumblar malware is also dynamically obfuscated (hidden), so hampering detection using traditional signature strings.
"Gumblar is arguably one of the most insidious threats facing both web surfers and website operators today", she said.
"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", she added.
Drawing on research from its latest monthly IT security report, ScanSafe claims that Gumblar is unique as the attackers are gaining access via stolen FTP credentials rather than using the usual code injection methods.
And, 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", said 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 2000 backdoored websites serving as actual malware hosts", she added.
"As a result, there is no single or few points at which to target efforts to shutdown the source of malware."