Although they are aware that paper mail can be forged, most users are blissfully unaware that email can also be forged. Forged mail can lead to a serious breach of security. Two points of vulnerability that require particular attention are the queue file and the SMTP interface of sendmail.
All versions of sendmail trust the files in the mail queue. They assume that only sendmail has placed files there. As a consequence, a poorly protected queue directory can allow the attacker to create mail that looks 100% authentic. This can be used to send forged mail, to append to system-critical files, or to run arbitrary programs as root or other users. Consider the following bogus qfAA00001 file for sending forged mail (qf files are described in Section 11.11):
V1 T829313834 P943442 $_root@yourhost S<root@yourhost> RPFD:george@yourhost H?P?return-path: <root@yourhost> Hmessage-id: <199604121257.GAA12601@yourhost> HFrom: root@yourhost HDate: Fri, 13 Dec 1996 05:47:46 -0700 HTo: george@yourhost HSubject: Change your Password Now!!
qf file causes mail to be sent to
george that appears in all ways to come from
root. There is nothing in this
qf file to indicate to the recipient (or to
sendmail) that the message is not authentic. Now
further suppose that the
df file (the message
body) contains the following text:
The system has been compromised. Change your password NOW! Your new password must be: Fuzz7bal Thank you, --System Administration