CVE-2026-45734 MyBB: Default CAPTCHA missing invalidation
Attackers can bypass CAPTCHA protections in MyBB by replaying valid responses, enabling automated abuse of forum features.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-45734?
CVE-2026-45734 is a medium-severity vulnerability in mybb, published on 18 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-837
Which versions are affected by CVE-2026-45734?
How do I fix CVE-2026-45734?
No fix has been published for CVE-2026-45734 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-45734 being exploited?
There is no known exploitation of CVE-2026-45734 in the wild and no public exploit code.
Sources
Also known as GHSA-jrrr-f3jw-mjmc.