Bypass of CAPTCHA Controls in MyBB Forum Software
CVE-2026-45734

5.3MEDIUM

Key Information:

Vendor

Mybb

Status
Mybb
Vendor
CVE Published:
18 August 2026

What is CVE-2026-45734?

MyBB, a widely used free and open-source forum software, has a vulnerability related to its built-in CAPTCHA implementation. Prior to version 1.8.40, the CAPTCHA did not effectively enforce single-use semantics, allowing remote attackers to exploit the system via a challenge replay attack. This flaw affects multiple functions, including contact.php and member account actions, where the CAPTCHA validation could be reused without invalidation. The reliability of security measures relies on unique challenge responses, but this oversight permits attackers to bypass essential security controls. The issue has been addressed and resolved in version 1.8.40.

Affected Version(s)

mybb < 1.8.40

References

https://github.com/mybb/mybb/security/advisories/GHSA-jrr...
x_refsource_CONFIRM
https://github.com/mybb/mybb/commit/c2ed54f9259b9ce05728a...
x_refsource_MISC
https://github.com/mybb/mybb/releases/tag/mybb_1840
x_refsource_MISC

CVSS V3.1

Score:
5.3
Severity:
MEDIUM
Confidentiality:
None
Integrity:
Low
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
None
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

.