CVE-2026-0999: Mattermost Server Auth Bypass Vulnerability

CVE-2026-0999 Overview

CVE-2026-0999 is an authentication bypass vulnerability affecting Mattermost Server that allows authenticated users to circumvent SSO-only login requirements. The vulnerability stems from improper validation of login method restrictions, enabling attackers to bypass Single Sign-On (SSO) authentication enforcement by leveraging userID-based authentication mechanisms.

This authentication bypass (CWE-303: Incorrect Implementation of Authentication Algorithm) represents a significant security concern for organizations that rely on SSO enforcement to maintain centralized access control and security compliance.

Critical Impact

Authenticated users can bypass SSO-only login requirements, potentially circumventing organizational authentication policies and security controls.

Affected Products

• Mattermost Server versions 11.1.x through 11.1.2
• Mattermost Server versions 10.11.x through 10.11.9
• Mattermost Server versions 11.2.x through 11.2.1

Discovery Timeline

• 2026-02-16 - CVE-2026-0999 published to NVD
• 2026-02-18 - Last updated in NVD database

Technical Details for CVE-2026-0999

Vulnerability Analysis

This vulnerability exists due to insufficient validation of authentication method restrictions within Mattermost Server. When organizations configure SSO-only authentication to enforce centralized identity management, the application fails to properly validate that all login attempts adhere to this policy.

The core issue lies in how Mattermost handles userID-based authentication pathways. While the system may be configured to require SSO authentication exclusively, alternative authentication code paths exist that accept userID-based credentials without verifying the configured login method restrictions.

Root Cause

The root cause is classified as CWE-303 (Incorrect Implementation of Authentication Algorithm). The authentication validation logic does not consistently enforce login method restrictions across all authentication endpoints. This creates a gap between the intended security policy (SSO-only) and the actual enforcement, allowing users who already have valid credentials to authenticate through non-SSO mechanisms.

Attack Vector

An attacker with valid Mattermost credentials can exploit this vulnerability by:

1. Identifying that the target Mattermost instance enforces SSO-only authentication
2. Crafting authentication requests that utilize userID-based authentication instead of the mandated SSO flow
3. Successfully authenticating to the system while bypassing SSO controls

This attack requires network access and valid user credentials (low privileges), but does not require user interaction. The vulnerability allows unauthorized modification of authentication flows, though it does not directly expose confidential data or cause service disruption.

The vulnerability mechanism involves improper validation within the login authentication flow. When SSO-only restrictions are configured, the system should reject all non-SSO authentication attempts. However, the userID-based authentication pathway does not properly check these restrictions before processing the login request. For detailed technical information, refer to the Mattermost Security Updates advisory (MMSA-2025-00548).

Detection Methods for CVE-2026-0999

Indicators of Compromise

• Authentication events showing successful logins via userID-based methods when SSO-only mode is enabled
• Login audit logs indicating authentication method mismatches with configured policies
• Unusual authentication patterns where users bypass expected SSO redirects

Detection Strategies

• Monitor authentication logs for login events that do not originate from the configured SSO identity provider
• Implement alerting for successful authentications that bypass SSO validation workflows
• Review Mattermost audit logs for authentication method anomalies

Monitoring Recommendations

• Enable verbose authentication logging in Mattermost Server configurations
• Correlate Mattermost authentication events with SSO identity provider logs to identify discrepancies
• Establish baseline authentication patterns and alert on deviations from expected SSO flows

How to Mitigate CVE-2026-0999

Immediate Actions Required

• Upgrade Mattermost Server to patched versions immediately
• Review authentication logs for evidence of SSO bypass exploitation
• Verify SSO-only configurations are properly enforced post-upgrade
• Audit user accounts for any unauthorized access during the vulnerability window

Patch Information

Mattermost has released security updates addressing this vulnerability. Organizations should upgrade to fixed versions that properly validate login method restrictions. Consult the Mattermost Security Updates page for specific patched version information and advisory MMSA-2025-00548.

Workarounds

• Implement additional network-level access controls to restrict authentication endpoints
• Deploy Web Application Firewall (WAF) rules to filter suspicious authentication requests
• Consider temporarily disabling alternative authentication methods at the infrastructure level until patches can be applied
• Enable additional authentication logging and monitoring to detect bypass attempts

# Review Mattermost authentication configuration
# Verify SSO-only settings in config.json
cat /opt/mattermost/config/config.json | grep -A 10 "ServiceSettings"

# Check for authentication method restrictions
grep -i "EnableSignUpWithEmail\|EnableSignInWithEmail\|EnableSignInWithUsername" /opt/mattermost/config/config.json

# Review recent authentication events
tail -f /opt/mattermost/logs/mattermost.log | grep -i "login\|auth\|sso"