CVE-2026-45734: Mybb

Medium severity, CVSS 5.3. EPSS: 0.6% chance of exploitation in the next 30 days.

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.

Affected products

• Mybb Mybb: before 1.8.40 (fixed in 1.8.40)

Published 2026-08-18. Last modified 2026-09-08.