SSD Advisory – vBulletin routestring Unauthenticated Remote Code Execution

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vBulletin, also known as vB, is a widespread proprietary Internet forum software package developed by vBulletin Solutions, Inc., based on PHP and MySQL database server. vBulletin powers many of the largest social sites on the web, with over 100,000 sites built on it, including Fortune 500 and Alexa Top 1M companies websites and forums. According to the latest W3Techs1 statistics, vBulletin version 4 holds more than 55% of the vBulletin market share, while version 3 and 5 divide the remaining percentage

An independent security researcher has reported this vulnerability to Beyond Security’s SecuriTeam Secure Disclosure program

Vendor response
We tried to contact vBulletin since November 21 2017, repeated attempts to establish contact went unanswered. At this time there is no solution or workaround for these vulnerabilities.

Vulnerability details
vBulletin contains a vulnerability that can allow a remote attacker to include any file from the vBulletin server and execute arbitrary PHP code.

An unauthenticated user is able to send a GET request to /index.php which can then trigger the file inclusion vulnerability with parameter routestring=.

The request allows an attacker to create a crafted request to Vbulletin server installed on Windows OS and include any file on the web server.

Listing of /index.php:

Let’s take a closer look on vB5_Frontend_Application::init() – Listing of /includes/vb5/frontend/application.php:

We can see that setRoutes() is called:

Listing of /includes/vb5/frontend/routing.php:

So if our routestring does not end with ‘.gif’, ‘.png’, ‘.jpg’, ‘.css’ or ‘.js’ and does not contain ‘/’ char vBulletin will call legacy() method from vB5_Frontend_Controller_Relay – /includes/vb5/frontend/controller/relay.php:

If we will check relay() from Api_Interface_Collapsed class – /include/api/interface/collapsed.php:

As we could see an attacker is not able to use ‘/’ in the $file so he cannot change current directory on Linux. But for Windows he can use ‘\’ as path delimiter and is able to specify any desired file (he can use ‘\..\’ trick as well) and it will be included by php.

If we want to include file with extension like ‘.gif’, ‘.png’, ‘.jpg’, ‘.css’ or ‘.js’ we will need to bypass the mentioned check in setRoutes() method. This can be easily done by adding dot (‘.’) or space (‘%20’) to the filename.

Proof of Concept
We can check if the server is vulnerable by sending the following GET request:

If the response is:

The server is vulnerable.

If we want to inject a php code to any file on the server we can use the access.log for example:

After that we can include access.log with our PHP code: