On 10/21/2004 the following vulnerability was reported to Microsoft.

Security Flaw with Digital signatures in Microsoft Outlook -
Emails in Microsoft Outlook digitally signed with S/MIME using either a commercial personal
certificate like Verisign or using a certificate issued by MS
Certificate Server can be altered. Outlook will not show any warnings
about the email being changed, the digital signature will still be
reported valid even though the message content has been modified and
parties involved in the signatures changed.
This is an extremely serious flaw as I can change any digitally signed
emails I want without Outlook ever noticing.

After several emails with Microsoft and CERT during the months that followed, no fixes have been issued to correct this security flaw. It is only now that I am making this information public after all my attempts to have Microsoft resolve the problem have failed.

The following are 2 digitally signed messages. The 1st one is a valid, unmodified email from Roberto Franceschetti (roberto@logsat.com) to support@logsat.com: (follow the hyperlinks for the email's source)

The following one has been "hacked" so that the sender now appears to be "George Bush" (president@whitehouse.com - note this is *not* whitehouse.gov). Note that Outlook states that the email is absolutely valid, and that the certificate is Valid and Trusted. This is most definitely not the case, as I've altered the original message to make it appear as a different person actually sent it. Imagine the scenario where a digital signature is supposed to unequivocally identify a sender,  but now this email that appears to be sent by "George Bush" appears legitimate, and a poor victim will trust it and send the hacker any confidential information he is asked for... (follow the hyperlinks for the email's source):

This 3rd email is yet another variation showing how a digitally signed email can further be forget without Outlook ever raising warning flags (follow the hyperlinks for the email's source):

The full emails with the conversations between myself, Microsoft and CERT can be found here. I hope that by making this information public all the users who rely on digital signatures will be aware of this severe security flaw in Microsoft Outlook, and will take other precautions to ensure the identity of users in digitally signed emails they receive.

Roberto Franceschetti
LogSat Software
roberto@logsat.com