Outlook Express is the standard e-mail client that is shipped with Microsoft Windows 9x/ME/NT.
The address book in Outlook Express is normally configured to make entries for all addresses that are replied to by the user of the mail client. An attacker may construct a message header that tricks Address Book into making an entry for an untrusted user under the guise of a trusted one. This is done by sending a message with a misleading "From:" field. When the message is replied to then Address Book will make an entry which actually replies to the attacker.
Situation: 2 good users Target1 and Target2 with addresses firstname.lastname@example.org and
email@example.com and one bad user Attacker, firstname.lastname@example.org. Imagine Attacker wants to get
messages Target1 sends to Target2. Scenario:
1. Attacker composes message with headers:
From: "email@example.com" <firstname.lastname@example.org>
Reply-To: "email@example.com" <firstname.lastname@example.org>
To: Target1 <email@example.com>
Subject: how to catch you on Friday?
and sends it to firstname.lastname@example.org
2. Target1 receives mail, which looks absolutely like mail received from
email@example.com and replies it. Reply will be received by Attacker. In this case
new entry is created in address book pointing NAME "firstname.lastname@example.org" to
3. Now, if while composing new message Target1 directly types e-mail
address email@example.com instead of Target2, Outlook will compose address as
"firstname.lastname@example.org" <email@example.com> and message will be received by Attacker.