{"id":"CVE-2026-45734","title":"MyBB is free and open source forum software","summary":"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 validati…","severity":"medium","cvss":5.3,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N","cwe":["CWE-837"],"published":"2026-08-18","updated":"2026-09-08","sourceUpdated":"2026-09-08T21:02:26.047","source":"NVD","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-45734","references":[{"url":"https://github.com/mybb/mybb/commit/c2ed54f9259b9ce05728a9e657169033fe4adffc","label":"security-advisories@github.com"},{"url":"https://github.com/mybb/mybb/releases/tag/mybb_1840","label":"security-advisories@github.com"},{"url":"https://github.com/mybb/mybb/security/advisories/GHSA-jrrr-f3jw-mjmc","label":"security-advisories@github.com"},{"url":"https://mybb.com/versions/1.8.40","label":"security-advisories@github.com"}],"tags":["nvd"],"epss":0.00404,"epssPercentile":0.3447,"ingestedAt":"2026-09-08T21:11:12.280Z","slug":"CVE-2026-45734","body":"## Overview\n\nMyBB 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.\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.","depth":"sunlit","depthScore":29,"depthScoreParts":{"impact":29.2,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}