<h3>Impact</h3>
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
<h3>Details</h3>
The public Registration workflow (member.php?action=do_register) accepts a hidden field question_id — expected to match the question session identifier (mybb_questionsessions.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.
<h3>Patches</h3>
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
<h3>References</h3>
- Release Notes: https://mybb.com/versions/1.8.(...)/
<h3>For more information</h3>
Go to mybb.com/security to report possible security concerns or to learn more about security research at MyBB.
<h3>Contact</h3>
The security team can be reached at security@mybb.com.