{"id":"CVE-2026-45734","aliases":["EUVD-2026-61014","GHSA-jrrr-f3jw-mjmc"],"status":"published","title":"MyBB: Default CAPTCHA missing invalidation","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.","severity":{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-11","score":0.00551,"percentile":0.4655,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":false},"cwes":["CWE-837"],"published":"2026-08-18T15:51:07Z","modified":"2026-10-28T02:31:14Z","products":[{"vendor":"mybb","product":"mybb"}],"references":[{"url":"https://github.com/mybb/mybb/security/advisories/GHSA-jrrr-f3jw-mjmc","tags":["x_refsource_CONFIRM","advisory"]},{"url":"https://github.com/mybb/mybb/commit/c2ed54f9259b9ce05728a9e657169033fe4adffc","tags":["x_refsource_MISC","fix"]},{"url":"https://github.com/mybb/mybb/releases/tag/mybb_1840","tags":["x_refsource_MISC","web"]},{"url":"https://mybb.com/versions/1.8.40","tags":["x_refsource_MISC","web"]},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/45xxx/CVE-2026-45734.json","tags":["advisory"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-45734","tags":["advisory"]}],"sources":{"cve":[{"id":"CVE-2026-45734","title":"MyBB: Default CAPTCHA missing invalidation","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.","status":"published","published":"2026-08-18T15:51:07.257Z","modified":"2026-08-19T14:43:38.996Z","cvss":[{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N","level":"medium","assessor":"cna"}],"cwes":["CWE-837"],"updated_at":"2026-11-02T13:46:32.755125Z"}],"euvd":[{"id":"EUVD-2026-61014","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.","status":"published","published":"2026-08-18T15:51:07Z","modified":"2026-08-19T14:43:38Z","cvss":[{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N","level":"medium"}],"updated_at":"2026-11-02T18:01:44.906351Z"}],"nvd":[{"id":"CVE-2026-45734","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.","status":"published","published":"2026-08-18T16:17:08.98Z","modified":"2026-09-08T21:02:26.047Z","cvss":[{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N","level":"medium","assessor":"secondary"}],"cwes":["CWE-837"],"updated_at":"2026-11-02T14:12:38.976695Z"}],"osv":[{"id":"CVE-2026-45734","title":"MyBB: Default CAPTCHA missing invalidation","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.","status":"published","published":"2026-08-18T15:51:07.257Z","modified":"2026-10-28T02:31:14.614043592Z","cvss":[{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N","level":"medium"}],"cwes":["CWE-837"],"updated_at":"2026-11-02T15:45:05.525666Z"}]}}