CVE-2026-46482Network attack vector

CAPTCHA bypass via fail-open question_id validation

Published Aug 18, 2026 · Updated Aug 18, 2026

CAPTCHA bypass in MyBB before 1.8.40 allows remote attackers to submit registrations via a crafted Security Question value. In member.php?action=do_register, the registration flow accepts a hidden question_id field and fails open when that identifier is blank, forged, or expired, so no question error is added before the request continues. The flaw is reachable without authentication on forums that use the text-based Security Question CAPTCHA, and the observed impact is limited to bypassing that anti-automation check during registration.

CVSS severityUnavailable
Unscored
EPSS probability0.33%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

CAPTCHA bypass in MyBB before 1.8.40 allows remote attackers to submit registrations via a crafted Security Question value. In member.php?action=do_register, the registration flow accepts a hidden question_id field and fails open when that identifier is blank, forged, or expired, so no question error is added before the request continues. The flaw is reachable without authentication on forums that use the text-based Security Question CAPTCHA, and the observed impact is limited to bypassing that anti-automation check during registration.

The record

CVE
CVE-2026-46482
Published
Aug 18, 2026
Updated
Aug 18, 2026
Vendor
MyBB
Product
MyBB
Classifications
CWE-636
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 18, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 18, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 1.8.40

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

MyBB · MyBBversion=< 1.8.40

Published CVSS scores

No CVSS assessment is available in this record.

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-636Not Failing Securely ('Failing Open')

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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