CVE-2026-45734
CVE-2026-45734 is a medium-severity vulnerability with a CVSS 3.x base score of 5.3. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-837.
Key facts
- Severity: Medium (CVSS 3.x base score 5.3)
- EPSS exploit prediction: 0% (34th percentile)
- Actively exploited: Not listed in CISA KEV
- Weakness: CWE-837
- Published:
- Last modified:
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.
Frequently asked questions
- What is 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.
- How severe is CVE-2026-45734?
- CVE-2026-45734 has a CVSS 3.x base score of 5.3, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity low, and availability none.
- Is CVE-2026-45734 being actively exploited?
- It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (34th percentile), an estimate of the probability of exploitation in the next 30 days.
- How do I fix CVE-2026-45734?
- Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
- When was CVE-2026-45734 published?
- CVE-2026-45734 was published on 2026-08-18 and last updated on 2026-08-19.
References
- 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
Other CWE-837 vulnerabilities
- CVE-2025-54315 — High (CVSS 7.1): The Matrix specification before 1.16 (i.e., with a room version before 12) lacks create event uniqueness.
- CVE-2024-11301 — Medium (CVSS 6.5): In lunary-ai/lunary before version 1.6.3, the application allows the creation of evaluators without enforcing a unique…
- CVE-2024-4629 — Medium (CVSS 6.5): A vulnerability was found in Keycloak. This flaw allows attackers to bypass brute force protection by exploiting the…
- CVE-2024-11717 — Medium (CVSS 6.3): Tokens in CTFd used for account activation and password resetting can be used interchangeably for these operations.…
- CVE-2025-62784 — Medium (CVSS 5.3): InventoryGui is a library for creating chest GUIs for Bukkit/Spigot plugins. Versions before 1.6.5 contain a…
- CVE-2025-62782 — Medium (CVSS 5.3): InventoryGui is a library for creating chest GUIs for Bukkit/Spigot plugins. Versions 1.6.3-SNAPSHOT and earlier…