The Gameover ZeuS malware is back. It was only six weeks ago that the takedown operation tried to cripple the bot. For the past month, the botnet formed by this malware was largely inactive, according to net security firm Sophos.

GameOver ZeuS was estimated to have infected over 500,000 machines worldwide. Its main function was to steal financial and personal information from compromised PCs. Gameover ZeuS was a common distribution mechanism for CryptoLocker prior to June's takedown operation.

Sophos reports that the new variant is distributed through widespread spam campaigns. The spam poses as online bank statements, the attachment is riddled with malware.

Criminal charges against a 30-year-old Russian national, Evgeniy Mikhailovich Bogachev, were brought at the time of the takedown. He was considered the person behind the bot.

According to Sophos, which has added protection against 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."