CVE-2026-39924 in Flarum
Summary
by MITRE • 08/05/2026
Flarum before 1.8.16 contains an improper session invalidation vulnerability that allows attackers who hold a valid session token to retain full account access after a victim changes their password, because the access_tokens table is never cleared on password change events. The TokensClearer::clearPasswordTokens() function only removes rows from the password_tokens table while leaving all active session cookies and API bearer tokens intact, including long-lived RememberAccessToken entries, and administrator-forced password resets via the user update endpoint are equally ineffective at revoking attacker-held sessions.
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Analysis
by VulDB Data Team • 08/05/2026
This vulnerability represents a critical session management flaw in Flarum versions prior to 1.8.16 that directly violates fundamental security principles of authentication state handling. The issue stems from an incomplete session invalidation mechanism where the system fails to properly terminate active sessions when users change their passwords, creating a persistent security risk that can be exploited by attackers who have obtained valid session tokens through various means including phishing attacks, session hijacking, or compromised credentials. The vulnerability specifically affects the TokensClearer::clearPasswordTokens() function which demonstrates a clear failure in implementing proper access token lifecycle management, leaving active sessions intact even when password changes occur.
The technical implementation flaw lies in the incomplete clearing of authentication tokens during password change operations, where only the password_tokens table is targeted for cleanup while ignoring the access_tokens table that contains active session cookies and API bearer tokens. This design oversight creates a fundamental gap in the security model, allowing attackers to maintain persistent access to victim accounts through long-lived RememberAccessToken entries that remain unaffected by the password change process. The vulnerability affects both regular user password changes and administrator-forced password resets executed via the user update endpoint, demonstrating that the flaw exists at the core session management logic rather than being limited to specific operational paths.
The operational impact of this vulnerability is severe as it undermines the principle of least privilege and account security enforcement mechanisms that should be automatically triggered when authentication credentials change. Attackers who have obtained valid session tokens can continue to operate with full privileges even after victims have changed their passwords, effectively nullifying the security benefit of password updates. This creates a window of opportunity for unauthorized access and potential data compromise, as demonstrated by the ATT&CK framework's privilege escalation techniques that leverage session persistence mechanisms. The vulnerability also aligns with CWE-613 which addresses insufficient session expiration and improper token handling during authentication state transitions.
The implications extend beyond simple unauthorized access to include potential data exfiltration, account takeovers, and lateral movement within affected systems when Flarum instances are part of larger network infrastructures. Organizations using vulnerable versions must consider that attackers who have compromised one user's session can maintain access even after the victim has taken corrective action by changing their password, creating a false sense of security during incident response activities. Effective mitigations require immediate patching to version 1.8.16 or later where the TokensClearer::clearPasswordTokens() function has been corrected to properly invalidate all active session tokens including those in the access_tokens table. Additionally, organizations should implement proactive monitoring for suspicious session activity and consider implementing session revocation mechanisms that automatically invalidate tokens upon password changes regardless of the specific change method used.
Organizations can enhance their defensive posture by implementing additional controls such as session timeout policies, multi-factor authentication requirements, and continuous monitoring of authentication events to detect anomalous behavior patterns that may indicate exploitation attempts. The vulnerability also highlights the importance of proper security testing including penetration testing and code reviews focused on authentication state management, particularly in web applications where session tokens are a fundamental component of access control systems. This issue serves as a reminder of the critical nature of session management in application security and the need for comprehensive security testing that covers not just authentication creation but also the complete lifecycle management of authentication tokens throughout their existence.