Medium 5.3Published Aug 18, 2026

CVE-2026-45734MyBB: Default CAPTCHA missing invalidation

Updated under an hour ago (2026-10-19 06:30 UTC)

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No exploitation reported and no EPSS estimate yet.

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Exploited in the last 30 days

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.

CVSS: 5.3 MEDIUM (v3.1, CNA)CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Weakness (CWE): CWE-837
Known exploited: Not in CISA's KEV catalog
EPSS (exploitation in the next 30 days): Not loaded for this CVE
Assigned by: GitHub_M
NVD analysis: Deferred
Record last updated: Aug 19, 2026
First seen by InfoSec Signals: Oct 9, 2026

Affected products

Affected products

mybb mybb mybb:mybb (from the name)	before 1.8.40	CNA record

References

https://github.com/mybb/mybb/security/advisories/GHSA-jrrr-f3jw-mjmcx-refsource-confirm
https://github.com/mybb/mybb/commit/c2ed54f9259b9ce05728a9e657169033fe4adffcx-refsource-misc
https://github.com/mybb/mybb/releases/tag/mybb_1840x-refsource-misc
https://mybb.com/versions/1.8.40x-refsource-misc

The CVE record on cve.org NVD's entry

The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.