MyBB: Default CAPTCHA missing invalidation

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-45734
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-08-18 16:17:08 UTC
Updated2026-09-08 21:02:26 UTC
DescriptionMyBB 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.

Risk And Classification

Primary CVSS: v3.1 5.3 MEDIUM from [email protected]

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

EPSS: 0.004040000 probability, percentile 0.337370000 (date 2026-09-08)

Problem Types: CWE-837 | CWE-837 CWE-837: Improper Enforcement of a Single, Unique Action

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary5.3MEDIUMCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
3.1CNADECLARED5.3MEDIUMCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Mybb Mybb affected < 1.8.40 Not specified

References

ReferenceSourceLinkTags
github.com/mybb/mybb/releases/tag/mybb_1840 [email protected] github.com
github.com/mybb/mybb/commit/c2ed54f9259b9ce05728a9e657169033fe4adffc [email protected] github.com
mybb.com/versions/1.8.40 [email protected] mybb.com
github.com/mybb/mybb/security/advisories/GHSA-jrrr-f3jw-mjmc [email protected] github.com
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