CVE-2026-3636: Mattermost Server Data Disclosure Flaw

CVE-2026-3636 Overview

CVE-2026-3636 is an information disclosure vulnerability in Mattermost Server that exposes team member role data to authenticated users without elevated permissions. The flaw stems from missing sanitization of team member data returned by various team API endpoints. An authenticated user can invoke these endpoints to retrieve role information for team members beyond what their permission level should allow. The issue is tracked under Mattermost Advisory ID MMSA-2026-00626 and is classified as [CWE-200] Exposure of Sensitive Information to an Unauthorized Actor.

Critical Impact

Authenticated low-privilege users can enumerate team member role assignments through Mattermost team API endpoints, exposing organizational structure and privileged account identities.

Affected Products

• Mattermost Server 11.6.x versions <= 11.6.0
• Mattermost Server 11.5.x versions <= 11.5.3
• Mattermost Server 11.4.x versions <= 11.4.4 and 10.11.x versions <= 10.11.14

Discovery Timeline

• 2026-05-22 - CVE-2026-3636 published to NVD
• 2026-05-22 - Last updated in NVD database

Technical Details for CVE-2026-3636

Vulnerability Analysis

The vulnerability resides in the team API response handling within Mattermost Server. When a user queries team membership endpoints, the server returns team member objects that include role assignments. The response serialization does not filter role fields based on the caller's permission level. As a result, users without elevated permissions receive role data that should be restricted to administrators or team admins.

The exposed data includes which accounts hold roles such as team_admin, system_admin, or other privileged designations. Attackers can use this information to identify high-value targets for follow-on attacks including credential phishing, social engineering, or account takeover attempts. The disclosure also reveals organizational hierarchy that supports reconnaissance of internal collaboration structures.

Root Cause

The root cause is improper output filtering in the team member API response path. Mattermost returns the full team member object structure without applying a permission-aware projection. The server trusts that role fields are non-sensitive, but role assignments constitute privileged metadata in multi-tenant deployments.

Attack Vector

Exploitation requires an authenticated session on the target Mattermost instance. The attacker invokes team API endpoints that list or retrieve team members and parses the returned JSON for role fields. No special tooling is required beyond standard HTTP clients and a valid user token. The vulnerability mechanism is described in the Mattermost Security Advisory.

Detection Methods for CVE-2026-3636

Indicators of Compromise

• Repeated API requests from a single authenticated user to team membership endpoints such as /api/v4/teams/{team_id}/members within short time windows.
• Enumeration patterns where one account queries members of multiple teams it does not actively participate in.
• Unexpected user agents or scripted clients accessing team API routes outside normal Mattermost web or mobile client traffic.

Detection Strategies

• Review Mattermost audit logs for high-volume calls to team member endpoints originating from non-administrative accounts.
• Correlate API access logs with user role assignments to identify low-privilege users issuing membership queries at scale.
• Baseline normal team API usage per user and alert on deviations that suggest systematic role enumeration.

Monitoring Recommendations

• Forward Mattermost application and reverse proxy logs to a central analytics platform for query and retention.
• Track HTTP GET request rates against /api/v4/teams/*/members endpoints, broken down by source user and source IP.
• Alert on accounts that retrieve member lists for more than a threshold number of teams within a defined interval.

How to Mitigate CVE-2026-3636

Immediate Actions Required

• Upgrade Mattermost Server to a fixed release above 11.6.0, 11.5.3, 11.4.4, or 10.11.14 depending on the deployed branch.
• Audit recent team API access logs to identify accounts that may have enumerated role data prior to patching.
• Review and tighten team membership policies to limit which users can join or query sensitive teams.

Patch Information

Mattermost has released fixed versions on the 11.6.x, 11.5.x, 11.4.x, and 10.11.x branches. Refer to the Mattermost Security Updates page for the specific patched build numbers and release notes corresponding to advisory MMSA-2026-00626.

Workarounds

• Restrict access to the Mattermost instance to trusted networks or VPN users while patching is scheduled.
• Reduce the number of accounts with team membership to high-sensitivity teams to limit the scope of exposed role data.
• Disable or restrict API token issuance for non-administrative users to reduce automated enumeration risk.

# Verify the running Mattermost Server version before and after upgrade
curl -s https://<mattermost-host>/api/v4/system/ping?get_server_status=true \
  -H "Authorization: Bearer <admin_token>" | jq .