Server-side polymorphism is a challenging problem for anti-malware software vendors. Much of today’s malware, such as the Storm worm, creates tens of thousands of variants each month, a development that has made many anti-virus software programs that use static signatures significantly less accurate.

Many of today’s worms and bots, which make up a large portion of the malware out on the Internet, launch from innocently infected Web sites. The criminal (or the criminal's automated program) breaks into the innocent Web site and injects code that then tries to infect every visitor running unpatched software. If the visiting user is fully patched, the site will prompt the user to install a “needed” program. The users, not realizing that their favorite trusted site is infected with malware, often follow these instructions.

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The first software that is downloaded to the user's machine and executed is a small malicious program with only one intention: silently install more malicious programs. The “downloader” installs anywhere from a handful to dozens of other malicious programs, each coming from a different infected or malicious Web host on the Internet. The eventual client-side program is intended to steal money (one way or another) or to be used in a distributed denial of service attack.

The programs and scripts used in these malicious exploits are often unscannable by traditional anti-virus scanners. The malware is not recognized by the signature recognition engine of the anti-malware program because the program never existed before. It’s a “one-off.” Every separate user connection to an infected Web server results in a different version of essentially the same malware program. 

Mutating malware
Variant and mutating malware is nothing new. It’s been around for more than two decades. What is new is that instead of the malware program itself doing the mutating, the pseudo-polymorphism (it really isn’t true polymorphism) is generated by the malicious server-side program. Until the user connects to the generating software, the malicious variant doesn’t exist. The malicious coding on the server creates a copy of the malware program, but re-encodes or packs the program, using a different packing engine. Even before the packing, it changes the names of the internal variables used, along with subroutine names and program logic locations. Program instructions and subroutines get moved around, encoded with arbitrary ASCII characters, and even the malicious payload is created on the fly.

Who cares whether anti-virus companies get the malicious sample and make a signature? The malware has never existed before and won’t exist again. Server-side polymorphism has created another challenge in the anti-virus world.