The criminals behind Licat have been quick to respond to MSN's updated network filters, and have already deployed a new method (which has already been seen in the wild) to bypass the filters.

What is it? Offline messages.

Windows Live Messenger (aka MSN Messenger 8) introduced the long awaited ability to send messages to offline users. Users of earlier versions of MSN Messenger can receive messages sent while they’re offline; they just can’t send messages to their contacts who are offline.

Why is this important? It turns out that messages sent to offline contacts in WLM aren’t being filtered in any way! This means that the attackers can send any message they want, provided it’s to offline users. We notified Microsoft of this filtering weakness yesterday.

We know that messages are being sent to offline users, but at the moment we’re not absolutely clear how this is being done - we haven't (yet) seen an IM-Worm which sends its messages to offline contacts.

While we were investigating the whole offline messages/ malware issue, we came across an interesting point. There are two ways to receive offline messages: either by a normal MSN pop-up window or by email. In the case of the ITW attack, the offline message arrived via email. In our testing we only received one offline message via email; after that, all offline messages were delivered by MSN IM pop-ups. (If anyone knows why this happened, please leave a comment or contact us at blog [at] viruslist [dot] com.)

It's to be hoped that Microsoft will fix this loophole as soon as possible. We'll also be keeping our eyes open for an IM-Worm which sends messages specifically to offline contacts.