FirstClass Internetgateway "stupidity"

[Mattias From <sirfrom () GEOCITIES COM>]

The email gateway included in FirstClass 5.50 can be tricked into
sending mail appearing to the users of the firstclass system as coming
from a local user on the server, including a priviliged user. Doing a
manual sending to the stmp-server specifying <username_on_system> as the
origin of the email will do but will be caught by the server
spamfilters, either discarding them or adding "Spam:" to the topic
depending on configuration. The requierment for a default configured
server not to view incoming mails as spam seems to be the presence of a
@ in the From: header line. Using a bogus address including an @
outside of the <> in the From: field and adding the shorthand adress
form <@>, which is expanded by the server to the adress specified with
MAIL FROM: during earlier smtp transaction will be delivered and marked
as coming from the specified user. The expanding behaviour is correct
iirc, not validating the origin (at all) is the problem.

Example:
-------------------------------------------------------------
220 mail.skynet.foo FirstClass ESMTP Mail Server v5.50 ready
MAIL FROM:<Admin>
250 Admin... Sender ok
RCPT TO:<user () skynet foo>
250 user () skynet foo... Recipient ok
DATA
354 Send your message, end with <CRLF>.<CRLF>
To: <user () skynet foo>
From: evil@socialengineer <@>
Subject: Gimme you password

Preferably now!
.
250 F1FEEACC Message accepted, transient identifier was 20
-------------------------------------------------------------

The above message will appear to user as a message from the local admin.
The ways to spot the message is in the firstclass "History" function
which specifies the mail as "Created by Admin" and "Routed from" an ip
adress, which normally does not exist. It's also spottable when the
headers, not shown by default, are viewed and that the info for the
Admin user is not opened correctly when accessed from that message.
Works on both Windows and MacOS versions of the server.
  //Mattias From