Vulnerability Description
MyBB is free and open source forum software. Prior to 1.8.40, the built-in CAPTCHA does not consistently enforce single-use semantics, allowing remote attackers to bypass CAPTCHA controls through challenge replay. The successful validation paths in contact.php, member.php?action=do_resendactivation, member.php?action=do_lostpw, member.php?action=do_emailuser, and sendthread.php?action=do_sendtofriend do not call captcha::invalidate_captcha() for the MyBB Default CAPTCHA selected by the captchaimage setting. A valid response can therefore be reused until a non-vulnerable endpoint invalidates it, an incorrect response is submitted, or the challenge expires. This issue is fixed in version 1.8.40.
CVSS Score
MEDIUM
Related Weaknesses (CWE)
References
- https://github.com/mybb/mybb/commit/c2ed54f9259b9ce05728a9e657169033fe4adffc
- https://github.com/mybb/mybb/releases/tag/mybb_1840
- https://github.com/mybb/mybb/security/advisories/GHSA-jrrr-f3jw-mjmc
- https://mybb.com/versions/1.8.40
FAQ
What is CVE-2026-45734?
CVE-2026-45734 is a vulnerability with a CVSS score of 5.3 (MEDIUM). MyBB is free and open source forum software. Prior to 1.8.40, the built-in CAPTCHA does not consistently enforce single-use semantics, allowing remote attackers to bypass CAPTCHA controls through chal...
How severe is CVE-2026-45734?
CVE-2026-45734 has been rated MEDIUM with a CVSS base score of 5.3/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-45734?
Check the references section above for vendor advisories and patch information. Review vendor security bulletins for remediation guidance.