CVE-2026-9824: Mattermost Server Auth Bypass Vulnerability

CVE-2026-9824 Overview

CVE-2026-9824 is a missing authorization vulnerability [CWE-862] in Mattermost Server. The /share-channel autocomplete handler fails to check the manage_shared_channels permission before returning data. Authenticated users without that permission can enumerate configured remote cluster connection metadata through slash command autocomplete.

The issue affects Mattermost Server versions 11.7.x through 11.7.2, 11.6.x through 11.6.4, and 10.11.x through 10.11.19. Mattermost tracks this issue as advisory MMSA-2026-00676.

Critical Impact
Any authenticated Mattermost user can enumerate remote cluster connection metadata, exposing federation topology and connected organization details that should be restricted to administrators with the manage_shared_channels permission.

Affected Products

Mattermost Server 11.7.x versions up to and including 11.7.2
Mattermost Server 11.6.x versions up to and including 11.6.4
Mattermost Server 10.11.x versions up to and including 10.11.19

Discovery Timeline

2026-07-13 - CVE-2026-9824 published to NVD
2026-07-13 - Last updated in NVD database

Technical Details for CVE-2026-9824

Vulnerability Analysis

Mattermost implements a slash command autocomplete feature that surfaces suggestions as users type commands. The /share-channel command autocomplete handler queries backend data about configured remote cluster connections and returns matching entries to the client.

The handler is intended to serve administrators managing shared channels between federated Mattermost instances. Access should be gated by the manage_shared_channels permission that governs the underlying feature. The vulnerable code path omits that check.

An authenticated user with a standard account can invoke the autocomplete endpoint and receive metadata describing every remote cluster connection configured on the server. That metadata reveals which external organizations the server federates with and identifiers used to establish those trusts.

Root Cause

The root cause is a missing authorization check [CWE-862] in the /share-channel autocomplete handler. The endpoint validates that the caller is authenticated but does not verify the manage_shared_channels permission before returning remote cluster data. Authentication is treated as sufficient authorization for a resource that requires elevated privilege.

Attack Vector

Exploitation requires network access to the Mattermost server and any valid user session. The attacker triggers the /share-channel slash command autocomplete flow and reads the returned suggestion payload. No user interaction beyond the attacker's own client actions is required. The attack yields confidentiality impact only. Integrity and availability are not affected because the handler is read-only.

See the Mattermost Security Updates advisory for additional technical detail.

Detection Methods for CVE-2026-9824

Indicators of Compromise

Repeated requests to the /share-channel autocomplete endpoint from user accounts that lack the manage_shared_channels permission
Autocomplete queries issued by accounts that have no operational reason to interact with shared channel functionality
Unusual volume of slash command autocomplete traffic originating from a single session or IP address

Detection Strategies

Review Mattermost audit logs for slash command autocomplete calls targeting /share-channel and correlate them with the caller's role and permission set
Alert on autocomplete requests where the responding metadata references remote cluster connections and the requester is not an administrator
Baseline normal slash command usage per role and flag deviations for the shared channel command family

Monitoring Recommendations

Forward Mattermost application and audit logs to a centralized logging platform for query and retention
Track the count of distinct users interacting with shared channel commands over rolling windows and investigate spikes
Monitor for enumeration patterns such as sequential autocomplete queries designed to iterate through remote cluster entries

How to Mitigate CVE-2026-9824

Immediate Actions Required

Upgrade Mattermost Server to a fixed release beyond 11.7.2, 11.6.4, or 10.11.19 as published on the Mattermost Security Updates page
Audit which user accounts have exercised the /share-channel autocomplete endpoint since the vulnerable versions were deployed
Review the list of configured remote cluster connections and rotate any secrets whose disclosure would materially affect the federation trust

Patch Information

Mattermost has released fixed versions addressed under advisory MMSA-2026-00676. Consult the Mattermost Security Updates portal for the specific patched build numbers that resolve CVE-2026-9824 and apply them following the vendor's upgrade guidance.

Workarounds

Restrict Mattermost account provisioning and disable unused accounts to reduce the population able to reach the vulnerable endpoint
Limit network exposure of the Mattermost server so that only trusted user populations can authenticate and issue slash commands
If shared channels are not in use, disable the shared channels feature to remove sensitive remote cluster metadata from the environment

bash

# Verify the running Mattermost Server version before and after upgrade
mattermost version

# Example package upgrade path on a Linux host
sudo systemctl stop mattermost
sudo tar -xvzf mattermost-<fixed-version>-linux-amd64.tar.gz -C /opt --overwrite
sudo systemctl start mattermost