MyBB 1.8.13 - Remote Code Execution

# Exploit Title: RCE in MyBB up to 1.8.13 via installer
# Date: Found on 05-29-2017
# Exploit Author: Pablo Sacristan
# Vendor Homepage: https://mybb.com/
# Version: Version > 1.8.13 (Fixed in 1.8.13)
# CVE : CVE-2017-16780

This RCE can be executed via CSRF but doesn't require it (in some special cases). The requirements are there shouldn't be a lock in the /install/ directory and then if you have access to the install directory you don't need CSRF, but if you don't then you need CSRF. I have included a patch and a description. The exploit will write PHP code to /inc/config.php which is then 'REQUIRE'd in most of the pages in MyBB, the PoC will just write lollol to the top of every page in MyBB. I also have an XSS but that I will report later.

There is a CSRF vulnerability in MyBB /install/index.php which can be used to inject PHP code into /inc/config.php which is then used in most of the pages (require MYBB_ROOT."/inc/config.php" is in most of the pages). 
  
The vulnerability exists in the table creation process for sqlite databases, this is because the Database Path is then inserted into the /inc/config.php file in line 11 as $config['database']['database'] = 'DB Path';
 
The vulnerability occurs because MyBB doesn't properly escape the Database Path, allowing an attacker to easily inject PHP by inserting a DB Path of : lol'; echo 'lol     this will not cause any parse errors since there will be a : ';    added at the end. Of course the attacker can easily just execute code in the server, getting backdoor access to the server easily.
 
A PoC would be to host a site like this:
<form name="x" action="http://localhost/install/index.php" method="post">
    
<input type="hidden" name='dbengine' value="sqlite">
<input type="hidden" name='config[sqlite][dbname]' value="lol'; echo 'lol">
<input type="hidden" name='config[sqlite][tableprefix]' value="mybb_">
<input type="hidden" name='action' value="create_tables">
                                        
</form>
 
<script>document.x.submit();</script>
 
 
And when a victim logged in as admin to a MyBB website visits this site they will have a "lollol" at the top of every page (or you can also make it do much more malicious things).
 
A simple patch would be to change /install/index.php:1410 to:
if(strstr($config['dbname'], "./") !== false || strstr($config['dbname'], "../") !== false || strstr($config['dbname'], "'") !== false || empty($config['dbname']))