CVE-2026-45734

Impact

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

Versions were read from the advisory text. No structured product data has been published for this record yet.

Weakness classification

References

• NVD record
• CVE.org record
• https://github.com/mybb/mybb/commit/c2ed54f9259b9ce05728a9e657169033fe4adffc
• https://github.com/mybb/mybb/releases/tag/mybb_1840
• https://github.com/mybb/mybb/security/advisories/GHSA-jrrr-f3jw-mjmc
• https://mybb.com/versions/1.8.40