EUVD-2026-61014Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Network-accessible unauthenticated exploit with no privileges needed; impact is integrity-only (CAPTCHA bypass enabling form abuse) with no confidentiality or availability consequence.
Primary rating from Vendor (GitHub_M).
CVSS VectorVendor: GitHub_M
Lifecycle Timeline
3DescriptionCVE.org
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.
AnalysisAI
CAPTCHA replay exploitation in MyBB prior to 1.8.40 allows unauthenticated remote attackers to bypass anti-automation controls by reusing a previously validated CAPTCHA challenge token across five distinct action endpoints. The root cause is missing calls to captcha::invalidate_captcha() after successful validation in contact.php, member.php (do_resendactivation, do_lostpw, do_emailuser), and sendthread.php (do_sendtofriend) when the built-in captchaimage setting is active. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Exploitation requires the target MyBB instance to have the built-in MyBB Default CAPTCHA enabled via the captchaimage configuration setting; instances using third-party CAPTCHA providers or with CAPTCHA disabled are not affected. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 5.3 Medium score (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N) accurately reflects the low-complexity, unauthenticated network exploitation path and the limited integrity-only impact. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | Full exploit scenario with step-by-step reproduction available after sign-in. |
| Remediation | Upgrade to MyBB 1.8.40, available at https://github.com/mybb/mybb/releases/tag/mybb_1840 and documented at https://mybb.com/versions/1.8.40. … Detailed patch versions, workarounds, and compensating controls in full report. |
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External POC / Exploit Code
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