Source: The Register
The servers of web ad network 24/7 Real Media were compromised and laced with references to a Dutch site which was hosting malware. The malware, dubbed Trojan.Zonebac, attempted a stealth install through a security hole in the RealPlayer software (for which a patch is now available as of Friday). "Symantec discovered the tainted ads on October 8. It remains unclear how many ads Real Media served or when the problem was corrected."