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" (https://www.first.org/cvss/calculator/3.1#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=do_register"" (https://github.com/mybb/mybb/blob/mybb_1839/member.php#L262-L307)) 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. 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" (https://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" (mailto:security@mybb.com).