Symantec customers who have not updated their antivirus software are being targeted by a self-propagating worm.
The company said the worm, dubbed Big Yellow by security vendor, eEye Digital Security, has been seen in a handful of attacks and is not considered to be a serious threat to most users.
It 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a flaw in Symantec's Client Security and AntiVirus Corporate Edition software, which was patched last May, so only out-of-date versions of the product are at risk.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 this attack, on Wednesday, senior director with Symantec Security Response, Vincent Weafer, said. "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 had counted about 70,000 compromised systems, the company's chief technology officer, Marc Maiffret, said.
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."
Though eEye put out a news release on Friday warning of the worm, the malware had not caused much concern in the security community because it was not widespread, a senior information security analyst at Cybertrust, Russ Cooper, said.
"This is [eEye] drumming up year-end press," he said.