Source: cve@mitre.org
MyBB (aka MyBulletinBoard) before 1.4.12 allows remote authenticated users to bypass intended restrictions on the number of [img] MyCodes by editing a post after it has been created.
MyBB (MyBulletinBoard) versions prior to 1.4.12 are vulnerable to a remote authenticated attack that allows users to bypass restrictions on the number of image tags ([img] MyCodes) within a post. This vulnerability could be leveraged to potentially cause a denial-of-service (DoS) by overwhelming the server with image requests or, in some scenarios, could be used for cross-site scripting (XSS) if the image tags are parsed unsafely.
Step 1: Authentication: The attacker must first have a valid, authenticated user account on the MyBB forum.
Step 2: Initial Post Creation: The attacker creates a new post, adhering to the [img] tag limit enforced during post creation.
Step 3: Post Editing: The attacker edits the previously created post.
Step 4: Payload Injection: The attacker adds more [img] tags to the post than the initial limit allowed.
Step 5: Post Submission: The attacker submits the edited post.
Step 6: Vulnerability Trigger: The MyBB software processes the edited post, now containing an excessive number of [img] tags, bypassing the intended restrictions.
The vulnerability stems from a flaw in how MyBB handles the enforcement of the maximum number of [img] tags within a post. The initial check for the tag count is performed during post creation. However, the system fails to re-validate this count when a user edits an existing post. This allows an authenticated user to bypass the restriction by adding more [img] tags after the post has been created. The root cause is a missing or insufficient check during the edit process. The lack of proper input validation and sanitization allows the attacker to inject an arbitrary number of image tags. This can lead to resource exhaustion on the server, potentially causing a DoS. Furthermore, if the image tags are not properly sanitized, it could lead to XSS vulnerabilities.
While no specific APTs are known to actively exploit this vulnerability, it could be incorporated into a broader attack campaign. This vulnerability is not listed on the CISA KEV catalog.
Monitor server logs for an unusual number of image requests originating from a single user or IP address.
Analyze forum post content for an excessive number of [img] tags, especially in edited posts.
Implement a web application firewall (WAF) rule to block or rate-limit requests containing a large number of [img] tags.
Monitor network traffic for suspicious patterns related to image requests, such as a sudden spike in requests or requests to unusual image hosts.
Upgrade to MyBB version 1.4.12 or later.
Implement server-side input validation to restrict the number of [img] tags allowed in a post, both during creation and editing.
Sanitize all user-supplied input, including the attributes of the [img] tags, to prevent XSS attacks.
Implement rate limiting on image requests to mitigate potential DoS attacks.
Regularly audit the forum's codebase for similar vulnerabilities and ensure that all input is properly validated and sanitized.