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 late last week 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..

Symantec's report stated that the Web site Tripod.com was one place an ad could be found last week triggering the exploit. As far as the ultimate source of the payload, Symantec's report stated it was still being analyzed, but researchers were certain it was downloading and executing a file from a newagetimes.am in the Netherlands, owned by an Amsterdam-based company.

The Trojan itself left on a victim's machine is probably the least exciting aspect of the malware distribution design, says Friedrichs, who says Zonebac is an older, well-known piece of malware. "The Trojan itself is not sophisticated, though it can disable security applications and modifies the registry."

While it's not known precisely why the attacker went through so much trouble to craft this complex a zero-day attack, it may have been simply to spread a spam relay. The larger mystery that remains a week later is how 24/7 Real Media's ads may have been a carrier or not.

24/7 Real Media says it's working with Symantec now to try to understand what happened. While Symantec's report originally stated the 24/7 Real Media ad server was compromised, Friedrichs says it's possible the exploit may have been carried out by someone paying for a Web ad that was tainted and purposely designed to play a role in the attack method.

"It looks like something occurred," said a 24/7 Real Media spokesman. "How it worked is the big question. We're working closely with Symantec to figure out what happened." The spokesman said 24/7 Real Media isn't completely sure its own ad-serving operations played any role at all but is making an effort to determine that.

In the DeepSight research report, Symantec says it sees evidence the exploit perpetrated by the attackers may have extended over the past year.