Older versions of the Internet JunkBuster Web proxy — which blocks Web banner ads — are reportedly being used to relay spam. Early versions of the product, though not the most recent, may be subject to attack if the administrator does not take precautions to block unauthorized access. In particular, version 2.0-1 of the product, as included and installed with old versions of Red Hat Linux, may be vulnerable.
If youre running the Internet JunkBuster proxy, be sure to look at the instructions at the link just above to make sure youre secure.