CVE-2026-18256: Mattermost Information Disclosure Vulnerability

CVE-2026-18256 Overview

CVE-2026-18256 is an authorization flaw in Mattermost that exposes private team metadata through the public team search function. Authenticated users without permission to list private teams can disclose the ID, name, and display name of private teams by submitting a modified team search request. The weakness maps to [CWE-863] Incorrect Authorization. Mattermost tracks this issue as advisory MMSA-2026-00748.

Critical Impact
Any authenticated Mattermost user can enumerate private team identifiers and names, undermining the confidentiality guarantees of private team workspaces.

Affected Products

Mattermost 11.10.x through 11.10.0
Mattermost 11.9.x through 11.9.1
Mattermost 11.8.x through 11.8.5 and 11.7.x through 11.7.9
Mattermost Advisory: MMSA-2026-00748

Discovery Timeline

2026-10-16 - CVE-2026-18256 published to the National Vulnerability Database (NVD)
2026-10-16 - Last updated in NVD database

Technical Details for CVE-2026-18256

Vulnerability Analysis

The vulnerability resides in the team search handler used by Mattermost's public team discovery feature. The server fails to re-check caller permissions against the list_private_teams authorization scope when processing a modified search request. As a result, private teams are included in the response set alongside public teams.

An authenticated attacker gains the team ID, name, and display_name fields for every private team the request matches. While channel contents and membership rosters remain protected, the leaked identifiers enable reconnaissance against sensitive projects, mergers, incident response channels, or executive workspaces hosted in Mattermost.

The issue is scoped to information disclosure. It does not grant join permissions, message access, or administrative privileges by itself.

Root Cause

The root cause is improper authorization enforcement [CWE-863] in the team search API. The handler applies the public-team visibility filter but omits a conditional check that would exclude private teams when the caller lacks the listing permission. The authorization decision depends on request parameters that the client controls, which allows the filter to be bypassed by altering the search query.

Attack Vector

Exploitation requires only valid user credentials on the target Mattermost instance. The attacker sends an authenticated HTTP request to the team search endpoint with modified query parameters, then parses the JSON response for private team entries. No user interaction, elevated privileges, or specialized tooling is required. See the Mattermost Security Updates advisory for the vendor description.

The vulnerability is exploitable over the network against any reachable Mattermost deployment running an affected build.

Detection Methods for CVE-2026-18256

Indicators of Compromise

Unusual volume of requests to the Mattermost team search API endpoint from a single authenticated session.
Team search requests containing non-default or malformed parameters inconsistent with the standard web client.
Audit log entries showing team listing responses that include private team IDs returned to non-privileged accounts.

Detection Strategies

Review Mattermost application logs and API gateway logs for team search calls that return responses containing private team identifiers.
Correlate authenticated session activity with team enumeration patterns, such as iterative queries with varied search strings.
Baseline normal team search behavior per user role, then alert on deviations from that baseline.

Monitoring Recommendations

Forward Mattermost audit logs and reverse-proxy access logs to a centralized SIEM for retention and query.
Create alerts for authenticated low-privilege users accessing the team search endpoint at elevated rates.
Monitor for the presence of private team metadata in API responses delivered to accounts without the private team listing permission.

How to Mitigate CVE-2026-18256

Immediate Actions Required

Upgrade Mattermost to a patched release above 11.10.0, 11.9.1, 11.8.5, or 11.7.9 as published in the vendor advisory MMSA-2026-00748.
Audit historical team search API logs to identify accounts that may have enumerated private teams prior to patching.
Review private team naming conventions and rename any team whose disclosed name alone constitutes sensitive information.

Patch Information

Mattermost has released fixed builds that correct the authorization check in the team search handler. Administrators should consult the Mattermost Security Updates page for the specific patched version matching their deployment branch and apply updates following standard change control procedures.

Workarounds

Restrict network access to the Mattermost API to trusted users where feasible until the patch is applied.
Enforce stricter role assignments so that only required accounts retain any team-listing permissions.
Temporarily disable or rate-limit the team search endpoint at the reverse proxy if upgrading cannot be scheduled immediately.

bash

# Example reverse-proxy rate limit for the Mattermost team search endpoint (nginx)
limit_req_zone $binary_remote_addr zone=team_search:10m rate=5r/m;

location /api/v4/teams/search {
    limit_req zone=team_search burst=3 nodelay;
    proxy_pass http://mattermost_upstream;
}