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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.

To exploit the vulnerability an attacker would send an email to a victim, within which is a link to a service provider's own server. This link would contain Javascript that executes when the page is loaded and opens a window that could navigate through the victim's inbox, sending messages with the malicious link to every email address it finds in the inbox.

The trick of this exploit is that since the malicious Javascript executes inside a page from the mail service's own server, no domain-bounding error is triggered when the Javascript is controlling the window with the victim's inbox.

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. ®

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