CVE-2026-7184: Mattermost Server Auth Bypass Vulnerability

CVE-2026-7184 Overview

CVE-2026-7184 is an information disclosure vulnerability in Mattermost Server affecting the Remote Cluster API. The flaw stems from improper sanitization of API responses during PATCH operations on the remote cluster endpoint. Authenticated users holding the manage_secure_connections permission can retrieve remote cluster authentication tokens by issuing a crafted PATCH request. Exposure of these tokens enables impersonation of trusted federated cluster connections. The issue is tracked under Mattermost Advisory ID MMSA-2026-00662 and classified as [CWE-201] Insertion of Sensitive Information Into Sent Data. Affected releases include Mattermost 11.6.x through 11.6.1, 11.5.x through 11.5.4, and 10.11.x through 10.11.15.

Critical Impact
Authenticated operators with secure connection management rights can extract remote cluster authentication tokens, enabling unauthorized access to federated Mattermost deployments.

Affected Products

Mattermost Server 11.6.x versions up to and including 11.6.1
Mattermost Server 11.5.x versions up to and including 11.5.4
Mattermost Server 10.11.x versions up to and including 10.11.15

Discovery Timeline

2026-06-12 - CVE-2026-7184 published to NVD
2026-06-18 - Last updated in NVD database

Technical Details for CVE-2026-7184

Vulnerability Analysis

The vulnerability resides in the Mattermost Remote Cluster API, which manages federated server-to-server connections. When a user issues a PATCH request to update a remote cluster configuration, the API returns the full remote cluster object in its response. The response payload includes the authentication token used to validate inbound traffic from the paired cluster. This token should be treated as a server secret and never returned to API consumers. Because the PATCH handler omits the field-stripping logic applied to other endpoints, the token leaks to any caller with the manage_secure_connections permission. An attacker can use the recovered token to impersonate the remote cluster, post messages, or pivot into federated channels.

Root Cause

The root cause is missing output sanitization in the PATCH response serializer for the remote cluster resource. Sensitive fields, including the shared authentication token, are not redacted before the JSON response is returned. This is a classic [CWE-201] exposure pattern, where sensitive data is sent in a response that should have been filtered.

Attack Vector

Exploitation requires network access to the Mattermost API and authenticated credentials with the manage_secure_connections permission. The attacker sends a PATCH request to the remote cluster endpoint with any valid modification payload. The server processes the update and returns the updated object, including the cleartext authentication token. No user interaction is required and the attack does not affect integrity or availability of the target system.

No verified public exploit code is available. See the Mattermost Security Updates advisory for additional technical context.

Detection Methods for CVE-2026-7184

Indicators of Compromise

PATCH requests to /api/v4/remotecluster/{remote_id} originating from accounts that do not routinely manage federation
Unexpected modifications to remote cluster records followed by outbound connections from new IP addresses using the leaked token
API audit log entries showing repeated PATCH calls to the remote cluster endpoint within a short time window

Detection Strategies

Review Mattermost audit logs for calls to the remote cluster PATCH endpoint and correlate with the requesting user identity
Alert on any account other than dedicated federation administrators invoking manage_secure_connections actions
Inspect response sizes and access patterns on the remote cluster API to identify anomalous enumeration behavior

Monitoring Recommendations

Forward Mattermost application and audit logs to a central SIEM and retain them long enough to investigate token reuse
Monitor authentication events on remote clusters for use of tokens from unexpected source IP addresses or geolocations
Track changes to the role and permission assignments that grant manage_secure_connections so privilege drift is detected promptly

How to Mitigate CVE-2026-7184

Immediate Actions Required

Upgrade Mattermost Server to a fixed release above 11.6.1, 11.5.4, or 10.11.15 as published in the Mattermost security advisory
Rotate all remote cluster authentication tokens after patching to invalidate any tokens that may have been disclosed
Audit accounts assigned the manage_secure_connections permission and remove the role from users who do not require it

Patch Information

Mattermost has released fixed versions through the Mattermost Security Updates portal under advisory MMSA-2026-00662. Administrators should consult the advisory for exact patched build numbers and follow the standard upgrade procedure for their deployment channel.

Workarounds

Restrict the manage_secure_connections permission to a minimal set of trusted system administrators until the patch is applied
Disable remote cluster federation if it is not in active use to remove the attack surface entirely
Place the Mattermost API behind network controls that limit administrative endpoints to trusted management networks

bash

# Example: list users granted manage_secure_connections via the Mattermost CLI
mmctl permissions role show system_admin | grep manage_secure_connections
mmctl permissions role show system_manager | grep manage_secure_connections

# After patching, rotate remote cluster tokens by recreating the secure connection
mmctl secure-connection delete <remote_id>
mmctl secure-connection create --name <name> --displayname <display>