CAPTCHA bypass via challenge replay
Published Aug 18, 2026 · Updated Aug 19, 2026
CAPTCHA bypass in MyBB before 1.8.40 allows remote attackers to reuse a solved challenge and submit protected forms. In contact.php, member.php, and sendthread.php, successful validation paths for the default captchaimage implementation omit captcha::invalidate_captcha(), so a valid response remains reusable across multiple requests and listed endpoints until it expires or is invalidated elsewhere. Exposure is limited to deployments using the built-in Default CAPTCHA on those flows; the documented impact is bypass of anti-automation checks rather than direct code execution or data access.
Summary
What happened
CAPTCHA bypass in MyBB before 1.8.40 allows remote attackers to reuse a solved challenge and submit protected forms. In contact.php, member.php, and sendthread.php, successful validation paths for the default captchaimage implementation omit captcha::invalidate_captcha(), so a valid response remains reusable across multiple requests and listed endpoints until it expires or is invalidated elsewhere. Exposure is limited to deployments using the built-in Default CAPTCHA on those flows; the documented impact is bypass of anti-automation checks rather than direct code execution or data access.
The record
- CVE
- CVE-2026-45734
- Published
- Aug 18, 2026
- Updated
- Aug 19, 2026
- Vendor
- MyBB
- Product
- MyBB
- Classifications
- CWE-837
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Aug 18, 2026CVE publishedPublication date reported by the CVE source.
- Aug 19, 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.
- Affected versionversion=< 1.8.40
What conditions does exploitation require?
What is affected?
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
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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