To allow for verification of the authenticity of mail messages, Microsoft Outlook Express supports digital signing of messages through S/MIME. A buffer overrun vulnerability lies in the code that generates the warning message when a particular error condition associated with digital signatures occurs.
By creating a digitally signed email and editing it to introduce specific data, then sending it to another user, an attacker could cause either of two effects to occur if the recipient opened or previewed it. In the less serious case, the attacker could cause the mail client to fail. If this happened, the recipient could resume normal operation by restarting the mail client and deleting the offending mail. In the more serious case, the attacker could cause the mail client to run code of their choice on the user’s machine. Such code could take any desired action, limited only by the permissions of the recipient on the machine.
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