Yahoo, the world's largest provider of email services, said on Monday that a software virus aimed at Yahoo Mail users had infected "a very small fraction" of its base of more than 200 million accounts.
The email virus, or worm, has been dubbed Yamanner and landed in Yahoo mailboxes bearing the headline "New Graphic Site." Once opened, the message infects the computer and spreads to other users listed in Yahoo users' email address books, security experts said.
The email containing the virus need only be opened--in contrast to most worms that are hidden in attachments and require users to take an additional step--to release the virus, according to computer security site Symantec.
The Sunnyvale, Calif.-based company advised users to update virus and firewall software on their computers and to block any email sent from the address "av3@ yahoo.com."
"We have taken steps to resolve the issue and protect our users from further attacks of this worm," Yahoo spokeswoman Kelley Podboy said in a statement.
"When we learn of email abuse, such as a worm or other online threat, we take appropriate action," she said. "(A) solution has been automatically distributed to all Yahoo Mail customers, and requires no additional action on the part of the user."
Yamanner, first detected by Yahoo and major computer anti-virus software makers earlier on Monday, was ranked as having a low threat level by Trend Micro and McAfee.
But Symantec considers the worm an "elevated threat," one step up from the lowest ranking in terms of relative danger.
Symantec's Security Response site suggested Yahoo Mail users might protect themselves by upgrading to the latest test version of the recently upgraded Yahoo Mail software.
"The worm cannot run on the newest version of Yahoo Mail Beta," Symantec's site said.
A Yahoo spokesman was not immediately available to comment on whether the company advised users to do this.
The email addresses are also sent to a remote online computer server, which may be used to run spam campaigns, experts said. The technical name of the worm goes by variants of 'JS.Yamanner.'