A mass-mail worm that exploits a vulnerability in Yahoo Inc.'s Web e-mail is making the rounds, but the impact appears to be low, security vendor Symantec Corp. said Monday.

The worm, which Symantec calls JS.Yamanner@m, is different from others in that a user merely has to open the e-mail to cause it to run, said Kevin Hogan, senior manager for Symantec Security Response. Mass-mail worms have usually been contained in an attachment with an e-mail note encouraging a user to open it.

The worm, written in JavaScript, takes advantage of a vulnerability that allows scripts embedded in HTML e-mail to run in the users' browsers. Yahoo users should be able to modify their settings to block the zero-day exploit, Hogan said.

Symantec rated the worm a Level 2 threat, one notch above its least harmful ranking. Hogan said the worm did not appear to be spreading widely, and he did not anticipate the threat level rising.

When activated, the worms then sends itself to other users in the victim's address book who also use Yahoo e-mail with the suffixes of @yahoo.com or @yahoogroups.com. The worm mimics a function within Yahoo's Web mail called "Quickbuilder," which allows a user to add contacts in an address book from received e-mail, Hogan said. The process, however, is transparent to the victim, he said.

The harvested e-mail addresses are sent to a remote server. Users of Yahoo Mail Beta do not appear to be affected, Symantec said.

The worm also opens a browser that displays a Web page that does not appear to contain malicious content.

Although Yahoo's Web e-mail has not been fixed, users are advised to update virus and firewall definitions and block any e-mail sent from [email protected]. The subject line of the e-mail with the worm says "New Graphic Site," and the body says "this is test."

Yahoo officials could not immediately be reached for comment.