A flaw in the web-based email services run by the likes of Yahoo! and Hotmail makes them conduits through which mass mailing email viruses can rapidly spread over the Internet.
That's the serious warning contained in a posting to the BugTraq security mailing list by a security researcher who said that cross-site scripting vulnerabilities make it possible to replicate Melissa-type worms through Web mail services.
Although the exploit requires a user to click on a link, the fact that a malicious email comes from a trusted source and contains a link to a familiar, trusted server works in favour of any worm created which makes use of the security loophole.
The vulnerability was discovered by a white-hat hacker called Matt Parcens who has posted an advisory on the issue and the steps that need to be taken to address the issue here.
Both Yahoo! and Hotmail were notified of the issue on May 23 but it's not known if a fix has been put in place, or which other services might be affected.
Jack Clark, European product manager at Network Associates, whose McAfee division provides the antivirus software used by Hotmail said that the threat looks "plausible" and that its researchers were investigating the issue with a view to developing a fix.
He questioned the motives of Parcens in posting details of the vulnerability so soon after notifying the firms involved which he compared to trying the locks on various door and then posting details of whose doors were left open on the Internet. ®
Melissa virus threatens to bring email to a halt
Reports of death of email viruses greatly exaggerated?
Rise in viruses within emails outpacing growth of email
Anna Kournikova bug drops harmlessly onto the Net
Homepage spreading faster than Kournikova worm
Users haven't learned any lessons from the Love Bug
Investors mis-interpret McAfee/MS .NET deal
Why Hotmail users get so much spam