The password-stealing ZeuS variant, Gameover, is now using encryption to get around perimeter security kit like firewalls and intrusion detection systems.
Malcovery's Gary Warner outlines the new behaviour of the malware at his blog, here, on the basis that the threat needed to be known beyond the circle of the company's customers.
Warner writes that the .EXE file associated with Gameover ZeuS should by now be spotted by up-to-date security, so the malware's authors have begun encrypting the file and distributing it as a non-executable .ENC file.
Of course, a .ENC file isn't executable (which is why it could get a pass mark from security systems), so the authors have to find some way to decrypt it at the target. They do this via a file included in the phishing e-mail that kicks off an attack: “the .zip file attached to the email has a NEW version of UPATRE that first downloads the .enc file from the Internet and then DECRYPTS the file, placing it in a new location with a new filename, and then causing it both to execute and to be scheduled to execute in the future”, Warner writes.
Boldizsár Bencsáth, from CrySys Lab in Hungary, explains the encryption here. It's not terribly sophisticated (since the purpose isn't to hide sensitive data, but merely to present security systems with a file format they'll ignore): the file is compressed, then XORd with a 32-bit key. The e-mail dropper that infects victims simply reverses this process. ®