Symantec anti-virus users are being targeted by a self-propagating worm, the company has warned, urging everyone to update their software.

The worm, dubbed Big Yellow by security vendor eEye, has been seen in a handful of attacks and is not considered to be a serious threat to most users, security vendors say.

The worm exploits a flaw in Symantec's Client Security and AntiVirus Corporate Edition software, which was patched last May, so only out-of-date versions of the product are at risk. The company's Norton products are not affected by the flaw.

Symantec first noticed some scanning activity on the Internet related to the attac on Wednesday, said Vincent Weafer, senior director with Symantec Security Response. "Since then it's gone to a background level," he said. "We have had three submissions locally from our customers."

However, according to eEye, infections are much more widespread than Symantec's data suggests. Since Thursday morning, eEye has counted about 70,000 compromised systems, said Marc Maiffret, the company's chief technology officer. Maiffret could not explain the discrepancy between eEye's and Symantec's data. "I don't know why they're saying that," he said.

The worm looks on port 2967 for unpatched clients. When it finds them, it installs its malicious software on the user's system, Weafer said.

A separate attack, which also exploited the same flaw, surfaced in late November, Weafer added. In that case, attackers focused on about a dozen educational institutions. "It occurred over a two-day period and then died down. This time around, with this new bot worm, we're not even seeing that [level of] activity."

eEye put out a news release Friday warning of the worm, but the malware has not caused much concern in the security community because it is not widespread, said Russ Cooper, a senior information security analyst at Cybertrust. "This is [eEye] drumming up year-end press," he said.