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Flaws in the filtering technology used by Web-based email services make it possible for hackers to smuggle viruses past defences.

Israeli security outfit GreyMagic Software warned today that this "severe security" vulnerability could allow attackers to run code of their choice, "simply by sending an email to an unsuspecting Hotmail or Yahoo! user". When the victim attempts to read this email, the code executes to potentially dire consequence (e.g. theft of the user's login and password, seizure of machines etc.).

The problem stems from a Cross-Site Scripting vulnerability involving IE. To blame is a new way to embed script involving an IE technology called HTML+TIME (based on SMIL), which is meant to add timing and media synchronization support to HTML pages.

The flaw weakens the ability of Web-based email services to screen this type of HTML content for malicious code. But users with up-to-date anti-virus scanners and personal firewalls are likely to be protected, even if hackers punch through that layer of defence.

GreyMagic has alerted Microsoft to this issue and worked with the company to fix the vulnerability in Hotmail. Hotmail is no longer vulnerable.

Unfortunately, all attempts by GreyMagic to contact Yahoo's security department failed; so Yahoo! webmail is still vulnerable.

GreyMagic warns that other web-based email systems may also be vulnerable. Users of these services may want to use a browser other than IE as a workaround, at least until a fix is in place. ®

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