CVE-2026-45734
MediumCVSS 5.3Summary
MyBB before version 1.8.40 does not consistently enforce single-use semantics for the built-in CAPTCHA, 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 default CAPTCHA.
Risk Assessment
An attacker can reuse the same CAPTCHA response multiple times, enabling automation of attacks such as spam or brute-force.
Recommendation
Update MyBB to version 1.8.40 or later, which includes a fix for invalidating CAPTCHA after use.
Original NVD description (English source)
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.