Let’s not forget that CHM files can be dangerous. They can contain embedded executables that get launched automatically when you open them. This post on SANS ISC details a particular malicious CHM file that was sent out via email. After some investigation, it was determined that the program it ran specifically targeted PGP keyrings.
The code searched for these files (.pkr and .skr) and copied them off to the attacker’s system. To really make use of a PGP keyring, you need the passphrase. Well, this is why the malware came bundled with a keylogger, just in case you happened to be using PGP while it was running. The ISC post also notes that it collected .doc files, which could be an attempt to harvest documents that users created to help them keep track of their passphrases.
I’m not sure if I see enough evidence to agree with the conclusion that the attacker was simply trying to map relationships between PGP users, but I guess that is a possibility. Do recent versions of PGP even use these same keyring files?
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