CVE-2026-45734
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.
Scoring
- Severity
- MEDIUM
- CVSS base score
- 5.3
- CVSS vector
- CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
- EPSS probability
- 0.40%
- CWE
- CWE-837
- Published
- 2026-08-18
- Last modified
- 2026-08-19
Affected products
- mybb mybb
Weakness type
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