CVE-2026-46482: MyBB: Security Question insufficient validation
Impact The registration component does not validate the text-based Security Question CAPTCHA correctly, allowing attackers to bypass the challenge via a specially crafted value.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Details The public Registration workflow (member.php?action=doregister) accepts a hidden field questionid — expected to match the question session identifier (mybbquestionsessions.sid) — and validates the challenge answer without a fail-closed fallback for invalid identifiers. If the value is blank, forged, or expired, the request continues without a question-related error.
Patches MyBB 1.8.(...) resolves this issue with the following changes:
- Commit: https://github.com/mybb/mybb/commit/ - .patch: https://github.com/mybb/mybb/commit/.patch
References - Release Notes: https://mybb.com/versions/1.8.(...)/
For more information Go to mybb.com/security to report possible security concerns or to learn more about security research at MyBB.
Contact The security team can be reached at security@mybb.com.
Affected Software
Event History
Frequently Asked Questions
Which deployments are exposed?
Sites using MyBB's public registration workflow with the text-based Security Question CAPTCHA are exposed. The affected request is member.php?action=do_register.
What does an attacker need to bypass the challenge?
No authentication or user interaction is required. An attacker can submit a registration request with a blank, forged, or expired question_id value so the Security Question is not rejected.
What is the available remediation?
Update to the release identified in the references as mybb_1840, which includes the referenced fix commit. If updating cannot happen immediately, the provided information does not identify a workaround.
How can I investigate whether the bypass was used?
Review registration requests for blank, forged, or expired question_id values that were accepted without a Security Question error. The data does not provide a definitive indicator of prior exploitation.