At the time of the advisory, there was no patch for the vulnerability. But by later on Monday, Yahoo said it had come up with a fix for the flaw, which it said had affected very few of its customers.
"We have taken steps to resolve the issue and protect our users from further attacks of this worm. The solution has been automatically distributed to all Yahoo Mail customers, and requires no additional action on the part of the user," a Yahoo representative said.
Both Yahoo and Symantec are encouraging people to update the antivirus definitions on their PCs.
Yamanner arrives in a Yahoo mailbox bearing the subject header "New Graphic Site." Once the message is opened, the computer becomes infected and the worm spreads itself to people on the Yahoo e-mail contact list. The harvested e-mail addresses are also sent to a remote online server, which Symantec suspects may use the information for spam campaigns.
Yamanner exploits the Yahoo flaw by enabling the scripts that are embedded in HTML e-mails to be run by the user's Web browser.
The worm, which was spotted in the wild early this morning, has hit the remote server more than 100,000 times, forwarding Yahoo e-mail addresses harvested from unsuspecting users, Turner said.
Although the worm is spreading quickly, and no patch has been issued, Symantec is rating the threat a "2." The security vendor uses a 1-to-5 rating system, with "5" as its most severe category.
"Antivirus definitions have been released for it, and Yahoo is working on a patch, so we don't want to cry wolf," Turner said. "Although there is the potential the worm will affect a larger number of people, for now to raise it to another (higher) level would be inappropriate."
He added it is premature to predict whether this worm will morph into other forms and attack other browser-based forms of e-mail, such as Google's Gmail.
Systems affected include Windows 2000, Windows 95, Windows 98, Windows Me, Windows NT, Windows Server 2003 and Windows XP, according to Symantec's advisory.