Microsoft has patched a bug in its Hotmail email service that attackers were exploiting to silently steal confidential correspondences and user contacts from unsuspecting victims.

The vulnerability was actively being exploited using emails that contained malicious scripts, Trend Micro researcher Karl Dominguez said Monday. Successful attacks required only that a Hotmail user open the malicious email or view it in a preview window. The commands embedded in the emails uploaded users' correspondences and user contacts to servers under the control of attackers without requiring the victim to click on links or otherwise take any action.

Trend Micro researchers learned of the in-the-wild attacks after a colleague in Taiwan received one of the booby-trapped emails. The email purported to be a security warning concerning the victim's Facebook account.

Trend first disclosed the bug on May 13. Monday's blog post said Microsoft has since plugged the hole, which resided in CSS, or cascading style sheet functionality, but didn't say when.

"The attack takes advantage of a script or CSS filtering mechanism bug in Hotmail," Dominguez wrote. "Microsoft has already taken action and updated Hotmail to fix the said bug."

The vulnerable code helped inject a character into a Hotmail filtering mechanism that changed the way it behaved. The result was a platform that ran arbitrary commands in a user's Hotmail login session.

It's unclear how many Hotmail users may have been affected by the exploits and whether Microsoft has adequately warned users they may have been compromised. Microsoft spokesman Bryan Nairn wouldn't say how many subscribers were targeted or when the patch was put in place. ®

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