A week after Symantec security researchers traced the elaborate course of a malware exploit -- apparently devised in the Netherlands -- to what may be a compromised ad server belonging to Internet advertising company 24/7 Real Media, the attack method isn’t fully understood.

The investigation started publicly last Friday when Symantec issued a 10-page DeepSight Threat Management System Threat Analysis written by Aaron Adams, Raymond Ball and Anthony Roe. The report accurately detailed the discovery of a zero-day attack based on a buffer overflow vulnerability in an ActiveX control in the popular desktop media player, RealPlayer from RealNetworks.

“It’s an ActiveX vulnerability, and this RealPlayer exploit runs JavaScript,” said Oliver Friedrichs, director of Symantec’s security response division. “The ActiveX control allows the malicious code to run, and it downloads a Trojan, one called Zonebac, which can disable security applications, modify the registry and perhaps later download more code. Just having RealPlayer on the desktop was enough.”

Its seriousness was understood well enough that the U.S. Computer Emergency Readiness Team issued a national security advisory this week about it, RealNetworks issued a patch to fix RealPlayer, and Symantec updated its desktop products to shield against the exploit on unpatched computers. Nevertheless, an aura of mystery still surrounds what role 24/7 Real Media's ad-serving network inadvertently may have played in the RealPlayer ActiveX exploit being disseminated across the Internet.

When Symantec researchers looked at logs and other data accumulated through its DeepSight honeypot network, they traced the path of the exploit back to 24/7 Real Media.

“What’s most interesting about the exploit is where it is hosted,” the Symantec researchers stated in their report. “The exploit itself is embedded in advertisements that were being served by 247realmedia.com. The redirection to the exploit page '83.149.65.105’ was accomplished through an IFrame embedded in each advertisement.”

According to the researchers, the iFrame in the Web ads silently loads an exploit located elsewhere on the Internet into victims' browsers without their knowledge. “This is a dangerous compromise in and of itself, however, the overall severity of the attack is revealed when the maze of URLs that lead to this advertisement is explored,” they stated..