Gumblar, a botnet that emerged as one of the Internet's largest earlier this year, is back, researchers said yesterday.

According to researchers at ScanSafe, a new iFrame injection is pointing once again to gumblar.cn -- the malware domain that originally earned Gumblar its name.

"The domain's reactivation occurred less than 24 hours ago, but it has ramifications that could stretch back for months," said ScanSafe researcher Gregg Conklin, in a blog. "Any sites compromised in the May Gumblar attacks that were not yet cleaned up (unfortunately an all-to-common occurrence) could now start becoming vectors of Gumblar infection once again. This is in addition to new compromises pointing to the newly-activated gumblar.cn and the already very active Gumblar compromises which are using compromised websites as malware hosts."

The Gumblar infection manifests in two ways, ScanSafe says. In some cases, the compromised sites may act as actual malware hosts. In other cases, the compromised sites are injected with iFrames that point to those malware hosts.

"In a typical Web malware outbreak, there are only a few malware hosts," Scansafe says. "But because the Gumblar attackers have this botnet of backdoored Websites at their disposal, there are now thousands of malware hosts. This makes stemming the flow of the attacks (i.e., shutting down the hosts) nearly impossible."

In October, 28.8 percent of all ScanSafe Web malware blocks were the result of the latest Gumblar attacks -- double the amount observed in the May outbreak, ScanSafe says. "The latest run of Gumblar has been highly successful for the attackers, all because the malware itself is being hosted on thousands of other compromised sites," Conklin says.

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