MyBB Threads to Link Plugin 1.3 - Cross-Site Scripting

# Exploit Title: MyBB Threads to Link Plugin v1.3 - Persistent XSS
# Date: 3/15/2018
# Author: 0xB9
# Contact: luxorforums.com/User-0xB9 or 0xB9[at]protonmail.com
# Software Link: https://community.mybb.com/mods.php?action=view&pid=1065
# Version: v1.3
# Tested on: Ubuntu 17.10
CVE: CVE-2018-10365


1. Description:
When editing a thread the user is given to the option to convert the thread to a link.


2. Proof of Concept:

Persistent XSS
- Edit a thread or post you've made
- At the bottom of the edit page in the Thread Link box input the following <a """><SCRIPT>alert("XSS")</SCRIPT>">
- Now visit the forum your thread/post exists in to see the alert.


3. Solution:
The plugin has since been removed after notifying the author.

Patch in line 83:
$thread['tlink'] = ($thread['tlink']);

to

$thread['tlink'] = htmlspecialchars_uni($thread['tlink']);