# CVE-2026-45734

## Summary

- **CVE ID:** CVE-2026-45734
- **Severity:** MEDIUM
- **CVSS Score:** 5.3 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N)
- **CWE:** CWE-837
- **Published:** Aug 18, 2026
- **Last Modified:** Aug 19, 2026

## 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.

## Affected Products

- mybb — mybb (< 1.8.40)

## References

- [CNA](https://github.com/mybb/mybb/security/advisories/GHSA-jrrr-f3jw-mjmc)
- [CNA](https://github.com/mybb/mybb/commit/c2ed54f9259b9ce05728a9e657169033fe4adffc)
- [CNA](https://github.com/mybb/mybb/releases/tag/mybb_1840)
- [CNA](https://mybb.com/versions/1.8.40)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.40%
- **EPSS Percentile:** 33.9

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-11._