Miscreants behind one variant of the ZeuS Trojan have outfoxed themselves in their attempts to outwit anti-virus analysts by releasing a variant of the malware that only infects high-performance PCs.
Security firms use automation and virtualisation technologies to cope with the growing volume of malware spewed out by cybercrooks every day. VXers are well aware of this and use virtual machine detection and anti-debugging code in their creations. The tactic is designed to frustrate security researchers and in so doing increase the time it takes to detect, develop and distribute anti-virus updates.
Users of the ZeuS crimeware toolkit are very much involved in this cat and mouse game between security researchers and cybercriminals.
But one particular group using the crimeware toolkit released a variant whose anti-debugging efforts are so aggressive it effectively assumes any machine whose CPU is running at lower than 2GHz must be running a debugger. As a result the malware only runs its malicious routines on high-performance machines, remaining inert on lower horsepower boxes.
Timo Hirvonen, a security analysis at F-secure, explains: "With a CPU below 2GHz the sample acts as if it is being debugged, aborts execution and does not infect the system. I tested the sample on an IBM T42 (1.86 GHz) notebook and the system was slow enough to avoid being infected."
This particular sample of ZeuS infects only high-speed machines, which might be useful if you wanted to build a premium spec botnet for code-cracking. Variants of ZeuS are much commonly used to capture online banking credentials before sending them off to cybercrooks, however, and from this perspective the sample is a dead loss.
ZeuS is a steadily evolving crimeware toolkit, sold on the black market for a few hundred dollar a licence. The apparent miscalculation with the particular Trojan captured by F-Secure that only infects high-performance machines has no bearing on the hundreds of other variants of ZeuS-spawned Trojans doing the rounds, unfortunately.
Assembly code fans can find a detailed explanation of the high-spec only malware sample's behaviour in a blog posting by F-Secure here. ®