Data-stealing malware pretends to be Microsoft IIS server module

The malware, which is dubbed “ISN,” hasn’t been widely seen, but its characteristics are interesting, wrote Josh Grunzweig, a Trustwave malware researcher, on a company blog.

ISN is a malicious DLL (dynamic link library), which is installed as a module for IIS, Grunzweig wrote. ISN’s installer contains four versions of the DLL, one of which is served up depending on whether a victim uses the 32- or 64-bit version of IIS6 or IIS7+.

“This module is of particular concern as it is currently undetectable by almost all anti-virus products,” Grunzweig wrote.

If ISN’s installer is detected, it’s usually through “general heuristic detection,” Grunzweig wrote, which means security software is looking at aspects of it that are suspicious and flagging it, such as if it is sending data to another server.

“I’m using this post as a way of notifying anti-virus vendors so that specific detections for this malware may be written,” he wrote, adding that he thinks the malware is “pretty neat.”

ISN collects data from POST requests, Grunzweig wrote. The stolen information is lifted from within IIS itself, which circumvents encryption, and then sent elsewhere. The malicious module can be configured to monitor information from specific URIs (uniform resource identifier), he wrote.

The malware has so far been “seen targeting credit card data on e-commerce sites, however, it could also be used to steal logins, or any other sensitive information sent to a compromised IIS instance,” he wrote.

“Overall, this malware does not appear to be widely spread and has only been seen in a few forensic case instances,” Grunzweig wrote. “However, the extremely low detection rate in collaboration with the malware’s targeted functionality makes this a very real threat.”