Code used to authenticate messages to Outlook Express users might be turned against them by attackers to run hostile code on victims' machines.

Microsoft's latest security advisory warns that a buffer overflow flaw in the Outlook Express S/MIME parsing functions "could enable system compromise". Redmond has issued a patch.

Yes, it's yet another critical security flaw, affecting only Outlook Express users. Other email clients, including Outlook, are immune.

Microsoft's advisory is vague about the precise cause of the problem. To authenticate messages, Outlook Express supports digital signing of messages through S/MIME.

The fault here lies in the code that generates the warning message when a particular error condition associated with digital signatures occurs.

Because of this buffer overflow bug, an attacker could create a digitally signed email and editing it to introduce specific data, which exploits the buffer overflow bug. This would either cause the email client to fail or, much more seriously, cause the mail client to run code of their choice on a user's machine.

This vulnerability could only affect messages that are signed using S/MIME and sent to an Outlook Express user. Credit for finding the flaw goes to Noam Rathaus of Beyond Security. ®