Hackers have hijacked a server operated by Internet advertising company 24/7 Real Media and are using it to seed legitimate Web sites with ads carrying attack code, Symantec said Friday.

Windows users who visited sites with the attacking ads were infected if they browsed with Microsoft's Internet Explorer and had RealNetworks' popular RealPlayer media player program installed on their PCs, Symantec said in an analysis written by three company researchers. This is the first time that malware has piggybacked on Internet ads served from a major advertising firm.

The attack should be a warning to the Web, said Andrew Storms, director of security operations at nCircle Network Security. "So much of the content we consume today comes from many syndication services," Storms said in an e-mail interview. "We trust that the content provided to us by Internet 'blue chips' is safe from malware.

"This should be a wakeup call for sites which offer syndicated content," Storms said. "They need to take a more active role in ensuring the security of [that] content."

Working off reports last week that RealPlayer and Internet Explorer could be exploited to infect Windows computers, Symantec researchers Aaron Adams, Raymond Ball and Anthony Roe used a compromised company honeypot to trace an attack back to 24/7 Real Media's server. Although Symantec didn't speculate on how the server was compromised, it did lay out the attack's progression.

How the hack worked

After they'd gotten access to the server, the attackers added code that embedded an IFrame in every advertisement. The invisible IFrame contained instructions to redirect any browser that rendered the ad to another, unauthorized IP address. In other words, users who surfed to a theoretically trustworthy site that contained ads inserted by New York-based 24/7 were, in fact, secretly shunted to the second, malicious site.

Script hosted on that second site sniffed users' machines to determine if they were vulnerable to the unpatched RealPlayer vulnerability before actually launching an attack, according to Symantec. "The script first tests the user-agent supplied by the browser ensuring that it is Internet 6 or 7 and the system is identified as NT 5.1 [Windows XP] or NT 5.0 [Windows 2000]," Adams, Ball and Roe said in a report. Other sniff tests included one to identify the version of RealPlayer on the vulnerable PC.

If the computer met the attack criteria, a second exploit script was executed, which in turn downloaded and installed a Trojan horse to the PC. The Trojan horse was a variation of "Zonebac," malware first detected last year that disables a slew of security software and lowers Internet Explorer's security settings, said the analysts. On Friday, Symantec called the original Zonebac "fairly unsophisticated" but added that the variant in the RealPlayer attack "retrieves information from numerous Web sites."

Symantec was not available over the weekend to answer questions about the nature of that information or to provide any other details of the attack.

"What's most interesting about the exploit is where it is hosted," the three researchers said. "The compromise of an ad server can greatly increase the effectiveness of the attack. It is so effective because it allows an attacker to target victims that are browsing trusted or well-known Web sites."

In the specific attack that Symantec monitored, the advertisement -- which was for job-hunting site Monster.com -- had been placed on a site hosted by Tripod.com, a Web hosting service owned by Lycos that offers both free and for-a-fee plans. "The Tripod.com Web site that triggered the breach on the DeepSight honeypot was 'xxxxxxxxx.tripod.com,' containing [an] embedded script ... which loaded the compromised advertisement and then in turn loaded the exploit," said the Adams, Ball and Roe report. "To emphasize the severity of this attack, [the ad script] is embedded and called in every Tripod.com user Web page (URLs formatted like 'name.tripod.com') at least," they added.