There's a pleasing symmetry about the latest security issue involving Outlook Express.

For the last couple of years Outlook (Lookout) Express failings have been exploited to infect users. So why not take advantage of its features to send viruses in such a way that they might fool detection by AV and content checking tools?

Well that's the gist of a new method of bypassing many SMTP-based content filter engines, unearthed by researchers at Beyond Security.

Using a rarely used feature called 'message fragmentation and re-assembly' (MFR), an attacker can send emails that will "bypass most SMTP filtering engines", Beyond Security reports.

This MFR feature, which allows Internet users to split up sent messages, helping surfers with slow connections to send smaller segments of a larger email in multiple emails, is supported by Internet standards (RFC 2046) but easily enabled on only one client - Outlook Express.

On Outlook Express the re-assembly feature is enabled by default, while the fragmentation feature can be enabled from a drop down menu.

The upshot of this is that virus writers might splice emails containing malware into smaller segments using Outlook Express that might slip past virus scanners or other content testing mechanisms because a recognisable virus signature might not appear in a particular email fragment.

Possibly.

This strikes us as a highly ineffective way to try to spread viruses, whose only use might be as a targeted attempt to infect a company which relies heavily on server-based AV scanners. Get beyond that shell and you're into a soft weakly protected belly, where all sorts of mischief can be wrought - at least in theory. In practice most corporates use desktop AV protection alongside server-based tools, and this will pick up any virus when the message is reassembled, at a user's PC.

Nonetheless a vendor solution to the vulnerability, involving including a reassembling agent at the server that prevents any non-reassembled message past, seems sensible - particularly for gateway AV scanners where the problem (such as it is) is most keenly focused.

Beyond Security has canvassed what's available. Trend Micro's InterScan VirusWall 3.5x for NT is affected, and the company has issued a patch. Likewise GFI MailSecurity for Exchange/SMTP 7.2 has been updated to detect this exploit as a "fragmented message".

All Symantec gateway products, by default, block multi-part MIME messages at the gateway, so there's not much of a problem there.

You can find Beyond Security's advisory, and a list of vendor responses, here.

Not all potentially affected vendors have responded as yet but GFI have put together a free test to check if you're vulnerable, available from the company's Security Testing Zone.