Attack Chain

1. Attacker crafts a malicious thread subject containing JavaScript code (e.g., <script>alert("XSS")</script>).
2. Attacker submits the crafted thread subject when creating a new thread on the MyBB forum.
3. The MyBB application stores the malicious subject in the database without proper sanitization.
4. A user visits the forum’s index page or any page that displays the thread’s subject.
5. The MyBB application retrieves the thread subject from the database and injects it into the HTML of the page.
6. The user’s browser parses the HTML and executes the injected JavaScript code.
7. The attacker’s JavaScript code performs malicious actions, such as stealing cookies or redirecting the user to a malicious website.

Impact

Successful exploitation of this XSS vulnerability can lead to various impacts, including session hijacking, where an attacker steals a user’s session cookie and gains unauthorized access to their account. Website defacement is also possible, where the attacker alters the appearance of the forum. In a targeted attack, the attacker could potentially gain control over the MyBB server itself, depending on the permissions of the user whose session is hijacked and the server configuration. Given the popularity of MyBB, a successful exploit could affect numerous forums and their users.

Recommendation

• Deploy the Sigma rule Detect MyBB XSS via Thread Title to identify potential exploitation attempts by detecting script tags in HTTP request parameters to thread creation endpoints.
• Inspect web server logs for HTTP requests containing <script> tags in the subject parameter when creating a new thread, as this is indicative of a potential XSS attack (see references for vulnerable parameter).
• Upgrade MyBB installations to a patched version that includes proper input sanitization to prevent XSS vulnerabilities.