CVE-2026-39923 in Flarum
Summary
by MITRE • 08/05/2026
Flarum before 1.8.16 contains a password reset token expiry bypass vulnerability that allows unauthenticated attackers to reuse expired password reset tokens by submitting them directly to the reset processing endpoint. The SavePasswordController::handle() method calls PasswordToken::findOrFail() without performing any expiry validation, allowing attackers to bypass the 24-hour token lifetime enforced only during form rendering and change any account's password to gain an authenticated session.
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Analysis
by VulDB Data Team • 08/05/2026
This vulnerability resides in Flarum versions prior to 1.8.16 and represents a critical authentication bypass flaw that undermines the security of the password reset mechanism. The issue stems from improper token validation logic within the SavePasswordController::handle() method, which fails to enforce the intended 24-hour lifetime restriction on password reset tokens. Attackers can exploit this weakness by capturing an expired token through legitimate means and then directly submitting it to the password reset processing endpoint without any additional authentication requirements.
The technical flaw manifests in the PasswordToken::findOrFail() method call which retrieves tokens from storage without validating their expiration status. This creates a scenario where tokens that should have expired after 24 hours remain functional, allowing unauthorized users to reset passwords for arbitrary accounts. The vulnerability specifically targets the server-side validation process rather than client-side rendering, meaning the token lifetime enforcement is inconsistent between the user interface and backend processing logic. This discrepancy enables attackers to bypass the intended security controls that are meant to limit password reset window to a 24-hour period.
The operational impact of this vulnerability is severe as it provides unauthenticated attackers with the ability to gain authenticated sessions for any user account within the system. Once an attacker successfully submits an expired token, they can change the target user's password and subsequently log in using their credentials. This creates a persistent threat vector that can be exploited repeatedly, potentially leading to complete account compromise and unauthorized access to sensitive data or system functionality. The vulnerability affects all users who have requested password resets, regardless of whether they were legitimate requests or malicious attempts.
From a cybersecurity perspective, this vulnerability aligns with CWE-613 which addresses insufficient session expiration and CWE-347 which covers improper certificate validation. The flaw also maps to ATT&CK technique T1531 related to "Modify System Executables" and T1078 related to "Valid Accounts" since it allows attackers to assume legitimate user identities through compromised authentication mechanisms. The vulnerability's exploitation requires minimal technical skill and can be automated, making it particularly dangerous in environments where users may not regularly monitor their account security. Organizations should prioritize immediate patching of affected systems and implement additional monitoring for suspicious password reset activities.
Mitigation strategies include applying the official Flarum 1.8.16 update which addresses the token validation logic by ensuring proper expiration checks are performed during token retrieval. Security teams should also consider implementing rate limiting on password reset requests to prevent automated exploitation attempts, while monitoring for unusual patterns in token usage. Additional defensive measures include enhancing user account monitoring for unauthorized password changes and considering multi-factor authentication as a compensating control to reduce the impact of compromised credentials. Organizations should conduct thorough security assessments of their authentication systems to identify similar validation gaps that might exist in other components of their infrastructure.