The Gameover ZeuS malware is back from the dead just six weeks after a takedown operations that aimed to put a stake through the heart of the botnet, which is linked to the even more infamous CryptoLocker ransomware.
International law enforcement acted against the crooks behind the Gameover ZeuS in early June. For the past month, the botnet formed by this malware was largely inactive, according to net security firm Sophos.
GameOver ZeuS, which is estimated to have infected more than 500,000 machines worldwide, is designed to steal financial and personal information from compromised PCs. Gameover ZeuS was a common distribution mechanism for CryptoLocker prior to June's takedown op. Cybercrooks behind the latest variant appear to have regressed to using older tricks involving infected email attachments and spam.
Now a new variant of Gameover ZeuS is being used to establish a zombie network. Sophos reports that the new variant is distributed through widespread spam campaigns, meaning the number of infections may already be large. Dodgy messages pose as online bank account statements, it said. The attachments of these messages are actually riddled with malware.
Some features of the old version have been dropped, including parts that were supposed to make it more sophisticated, in a move towards greater simplicity. In particular, the latest variant of the malware swaps P2P for fast-flux communication.
The latest variant of the malware tries phoning home to 1,000 domain names per day in order to receive command-and-control instructions. The crooks seem to be leaving it until the last minute to register domains they intend to use, according to an analysis of the latest variant by James Wyke, a senior threat researcher at Sophos.
The Gameover ZeuS takeover operation is June was accompanied by criminal charges against 30-year-old Russian national Evgeniy Mikhailovich Bogachev, who is suspected of running the botnet.
"We can't yet say whether this new variant is the old guys back... or someone completely new who acquired the source code," according to Sophos, which has added detection for the variant to its enterprise-focused security software packages.
The security software maker adds that it's too early to say whether the comeback campaign is likely to prove successful. Peter Kruse, a partner and eCrime specialist at Danish security consultancy CSIS, estimated there were 1,000 infections of machines contaminated with the new variant as of last Friday (11 July). ®