Over the past year, we’ve seen a huge jump in the number of mass downloader spyware. These small executable files have just one job, and they do it very well: They pull down huge numbers of additional installers, which in turn place a large number of password stealing Trojans, ad-clickers, and still more downloaders on the unfortunate victim’s PC.
The trend appears to be that most of the servers from which these phishing Trojans originate are registered within China’s .cn top-level domain, and the phishers themselves target (mostly) the login details for online multiplayer videogames played, primarily, in China, and in some cases, more widely in Asia.
Putting aside the rationale for what the phishers target (the goal may be purely financial, but that’s a discussion for another time), what’s really interesting is how the techniques to massively infect a victim’s PC have evolved, possibly to avoid network-based signature detection techniques that can identify Windows executable files while they’re traveling over the wire. It also seems that the various groups appear to compete with one another, even going so far as to block the domains used by competing groups’ downloaders once they’ve infected the machine.
So not long ago, another interesting mass downloader development seemed to drop into my work queue. These downloaders pull down bitmap images — not just executables with a different file extension, but real graphics files — then convert the color data into binary code, which transforms the data in the picture file into a small executable phisher installer.