CVE-2026-3590: Mattermost Auth Bypass Vulnerability

CVE-2026-3590 Overview

CVE-2026-3590 is a race condition vulnerability affecting multiple versions of Mattermost that fails to enforce atomic single-use consumption of guest magic link tokens. This security flaw allows an attacker with access to a valid magic link to establish multiple independent authenticated sessions via concurrent requests, effectively bypassing the intended single-use restriction of these authentication tokens.

Critical Impact

Attackers can exploit this race condition to establish multiple unauthorized sessions from a single guest magic link, potentially leading to unauthorized access persistence and session hijacking scenarios.

Affected Products

• Mattermost 10.11.x versions <= 10.11.12
• Mattermost 11.5.x versions <= 11.5.0
• Mattermost 11.4.x versions <= 11.4.2
• Mattermost 11.3.x versions <= 11.3.2

Discovery Timeline

• 2026-04-15 - CVE-2026-3590 published to NVD
• 2026-04-15 - Last updated in NVD database

Technical Details for CVE-2026-3590

Vulnerability Analysis

This vulnerability is classified under CWE-367 (Time-of-Check Time-of-Use Race Condition). The core issue lies in how Mattermost handles the consumption of guest magic link tokens during the authentication process. When a guest magic link is used for authentication, the system should atomically verify the token's validity and mark it as consumed in a single operation to prevent reuse.

However, the affected versions fail to implement this atomic operation correctly. The vulnerability creates a window of opportunity between the time the token is validated (time-of-check) and when it is marked as consumed (time-of-use). During this brief interval, an attacker can send multiple concurrent authentication requests using the same magic link token, each of which may succeed in creating an independent authenticated session.

The network-accessible nature of this vulnerability means attackers do not require any special privileges or user interaction to exploit it once they have obtained a valid guest magic link.

Root Cause

The root cause is a Time-of-Check Time-of-Use (TOCTOU) race condition in the guest magic link token validation and consumption logic. The system checks whether a token is valid and then separately marks it as used, without proper synchronization or atomic transaction handling. This non-atomic sequence allows concurrent requests to pass the validity check before any single request marks the token as consumed.

Attack Vector

The attack is conducted over the network by an attacker who has obtained access to a valid guest magic link (through interception, social engineering, or other means). The attacker sends multiple simultaneous HTTP requests to the authentication endpoint, each containing the same magic link token. Due to the race condition, multiple requests can successfully authenticate before the token is invalidated, resulting in the creation of several independent authenticated sessions.

The attack workflow involves capturing or obtaining a guest magic link, then rapidly sending concurrent authentication requests to the Mattermost server. The timing window, while brief, is exploitable with modern networking tools that can send parallel HTTP requests with sub-millisecond timing precision.

Detection Methods for CVE-2026-3590

Indicators of Compromise

• Multiple authenticated sessions created within milliseconds using the same guest magic link token
• Unusual spike in concurrent authentication requests from single or similar source IPs
• Multiple active sessions for the same guest user with overlapping creation timestamps
• Authentication logs showing rapid successive successful logins using identical magic link identifiers

Detection Strategies

• Monitor authentication logs for multiple successful logins using the same magic link token identifier
• Implement anomaly detection for concurrent authentication requests targeting guest magic link endpoints
• Configure alerting for multiple session creations from single guest invitations within short time windows
• Review session management logs for duplicate token usage patterns

Monitoring Recommendations

• Enable detailed logging for guest authentication endpoints to capture token usage timestamps
• Set up real-time monitoring for authentication endpoint request rates and patterns
• Implement session audit logging to track creation timestamps and correlate with magic link tokens
• Configure alerts for any instance where a single magic link results in multiple active sessions

How to Mitigate CVE-2026-3590

Immediate Actions Required

• Upgrade Mattermost to patched versions that address this race condition vulnerability
• Audit existing guest sessions and revoke any suspicious multiple sessions created from single invitations
• Review authentication logs for evidence of past exploitation attempts
• Consider temporarily disabling guest magic link functionality until patches are applied

Patch Information

Mattermost has released security updates addressing this vulnerability. Organizations should upgrade to versions beyond the affected ranges: 10.11.12, 11.5.0, 11.4.2, and 11.3.2. For detailed patch information and the latest secure versions, refer to the Mattermost Security Updates page. The advisory reference for this vulnerability is MMSA-2026-00624.

Workarounds

• Disable guest magic link invitations and use alternative authentication methods until patching is complete
• Implement network-level rate limiting on authentication endpoints to reduce the exploitability window
• Enable session monitoring and configure automatic session termination for anomalous duplicate sessions
• Restrict guest account creation permissions to reduce the attack surface

# Example: Rate limiting configuration at reverse proxy level
# Limit concurrent requests to authentication endpoints
# nginx configuration example
location /api/v4/users/login {
    limit_req zone=auth_limit burst=5 nodelay;
    limit_req_status 429;
    proxy_pass http://mattermost_backend;
}