Outlook Express version 5.50 and 6.0 contains a security vulnerability in the handling of S/MIME certificates which allows arbitrary code execution when inspecting a S/MIME signed message.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/30500/6D00B005PU.html
Source: https://packetstormsecurity.com/files/30500/6D00B005PU.html.html