CVE-2026-45734 in MyBB
Summary
by MITRE • 08/18/2026
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.
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Analysis
by VulDB Data Team • 08/18/2026
MyBB is a widely deployed free and open-source forum software that facilitates community discussions through threaded conversations. A significant security flaw was identified in versions prior to 1.8.40 regarding the implementation of its built-in CAPTCHA system, specifically concerning the enforcement of single-use semantics for challenge-response mechanisms. The vulnerability stems from an inconsistency in how different endpoints within the application handle the lifecycle of a generated CAPTCHA token. While some parts of the software correctly invalidate tokens after use to prevent reuse, several critical user-facing paths fail to execute this necessary cleanup procedure when using the default MyBB CAPTCHA engine selected via configuration settings.
The technical flaw resides in specific PHP files that process form submissions requiring human verification. These include contact.php for general inquiries, member.php with actions such as do_resendactivation for account recovery emails, do_lostpw for password resets, and do_emailuser for private messaging initiation, as well as sendthread.php when initiating the do_sendtofriend action to share threads via email. In these scenarios, although a valid CAPTCHA response is validated against the server-side challenge, the application fails to call the captcha::invalidate_captcha() function immediately after successful validation. This omission means that the token remains active and associated with its original challenge in the session or database state until it expires naturally due to time limits or is invalidated by another non-vulnerable endpoint within the same user session.
This architectural oversight allows remote attackers to perform CAPTCHA replay attacks, a technique categorized under CWE-347 Improper Verification of Cryptographic Signature and aligned with MITRE ATT&CK tactic T1621 Multi-Factor Authentication Interception or more broadly as part of automated attack patterns where human verification is bypassed. By capturing the HTTP response containing the valid CAPTCHA token from one request, an attacker can reuse this same token in subsequent requests to different endpoints within the application before it expires. This effectively neutralizes the protective purpose of the CAPTCHA, which is designed to distinguish between legitimate human users and automated bots or scripts.
The operational impact of this vulnerability is substantial for administrators relying on MyBB's default security configurations. Attackers can leverage this flaw to automate actions that are typically restricted by human verification controls. This includes spamming contact forms with malicious content, flooding password reset emails which may lead to account enumeration or resource exhaustion, sending unsolicited private messages via the email interface, and potentially facilitating brute-force attacks on user accounts if combined with other vulnerabilities. The ability to reuse tokens significantly lowers the barrier for automated abuse tools, allowing them to bypass rate limiting and human-centric controls that are essential for maintaining forum integrity and preventing spam.
Mitigation strategies primarily involve upgrading the MyBB installation to version 1.8.40 or later, where this logic error has been corrected by ensuring that captcha::invalidate_captcha() is called consistently across all relevant endpoints after successful validation. For organizations unable to upgrade immediately due to compatibility constraints with custom plugins or themes, a temporary workaround involves manually modifying the source code of the affected files to include the invalidation call post-validation. Additionally, administrators should consider implementing additional layers of defense such as IP-based rate limiting for sensitive actions like password resets and email sending, which can mitigate the impact even if CAPTCHA bypass occurs. Regular security audits and monitoring for unusual patterns in form submissions can also help detect potential exploitation attempts while patches are being applied.