{"apiVersion":"1.0","identifier":"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.","publishedAt":"2026-08-18T16:17:08","lastModifiedAt":"2026-08-19T15:17:04","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-45734","cvssScore":5.3,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N","epssProbability":0.00404,"riskScore":0.55,"affectedProduct":"MyBB","affectedVersions":"<1.8.40","vulnerabilityType":"Web app","operatingSystems":[],"links":{"self":"https://www.redsauce.net/api/cves/CVE-2026-45734","webPages":{"es":"https://www.redsauce.net/es/cves/CVE-2026-45734","en":"https://www.redsauce.net/en/cves/CVE-2026-45734","fr":"https://www.redsauce.net/fr/cves/CVE-2026-45734","pt":"https://www.redsauce.net/pt/cves/CVE-2026-45734","de":"https://www.redsauce.net/de/cves/CVE-2026-45734","sk":"https://www.redsauce.net/sk/cves/CVE-2026-45734","el":"https://www.redsauce.net/el/cves/CVE-2026-45734"},"source":"https://nvd.nist.gov/vuln/detail/CVE-2026-45734"}}