CVE-2026-2458: Mattermost Server Info Disclosure Flaw

CVE-2026-2458 Overview

CVE-2026-2458 is a Missing Authorization vulnerability (CWE-862) affecting Mattermost Server. The vulnerability exists due to improper validation of team membership when searching channels, allowing a removed team member to enumerate all public channels within a private team via the channel search API endpoint.

This authorization bypass represents a significant information disclosure risk in enterprise collaboration environments where team privacy boundaries are critical for organizational security. The flaw in the channel search API fails to verify that the requesting user still has valid team membership before returning search results.

Critical Impact

Removed team members can enumerate public channels within private teams they no longer have access to, potentially exposing sensitive organizational structure and channel naming conventions.

Affected Products

• Mattermost Server versions 11.3.x <= 11.3.0
• Mattermost Server versions 11.2.x <= 11.2.2
• Mattermost Server versions 10.11.x <= 10.11.10

Discovery Timeline

• 2026-03-16 - CVE-2026-2458 published to NVD
• 2026-03-18 - Last updated in NVD database

Technical Details for CVE-2026-2458

Vulnerability Analysis

This vulnerability stems from insufficient authorization checks in the Mattermost channel search API endpoint. When a user is removed from a team, their access tokens and session data may still be valid for a period of time. The channel search functionality fails to re-validate team membership before processing search queries, creating a window where removed users can continue to access team resources.

The vulnerability is exploitable over the network with low attack complexity, requiring only valid authentication credentials (low privileges). The impact is limited to confidentiality exposure of channel information within private teams, with no direct integrity or availability impact.

Root Cause

The root cause is a missing authorization check (CWE-862) in the channel search API handler. The endpoint verifies that the user is authenticated but fails to confirm that the authenticated user is still an active member of the team being searched. This represents a broken access control pattern where authentication is checked but authorization is incomplete.

The vulnerable code path likely processes the search request without querying the team membership table to verify current membership status, instead relying on cached or stale authorization data.

Attack Vector

An attacker who was previously a member of a private team can exploit this vulnerability through the following attack flow:

1. The attacker obtains valid authentication credentials while they are a team member
2. The attacker is subsequently removed from the private team by an administrator
3. Using their still-valid session or API token, the attacker sends requests to the channel search API endpoint
4. The API returns search results for public channels within the private team without validating current membership
5. The attacker can enumerate channel names, potentially revealing sensitive project names, team structures, or organizational information

The attack requires network access and valid authentication but does not require user interaction or elevated privileges. The scope is unchanged, meaning the impact is contained to the vulnerable component.

Detection Methods for CVE-2026-2458

Indicators of Compromise

• API requests to the channel search endpoint from users who are no longer team members
• Unusual patterns of channel search queries from accounts with recently revoked team memberships
• Authentication logs showing continued API activity from users after team removal events
• Search API requests targeting private teams from external IP addresses

Detection Strategies

• Correlate team membership removal events with subsequent API activity from the removed user accounts
• Monitor channel search API endpoints for requests from users not present in current team membership tables
• Implement anomaly detection for users accessing team resources after deprovisioning events
• Review audit logs for channel enumeration patterns that span multiple private teams

Monitoring Recommendations

• Enable comprehensive API logging for all channel search endpoint requests
• Configure alerts for team membership changes and correlate with subsequent access patterns
• Deploy SentinelOne Singularity XDR to monitor for post-authentication authorization bypass attempts
• Establish baseline API usage patterns to detect anomalous channel search behavior

How to Mitigate CVE-2026-2458

Immediate Actions Required

• Upgrade Mattermost Server to a patched version as specified in the security advisory
• Review team membership removal procedures and ensure sessions are invalidated upon team removal
• Audit recent team removal events and check for suspicious post-removal API activity
• Consider invalidating all active sessions for users who have been removed from teams

Patch Information

Mattermost has released security updates to address this vulnerability. Organizations should upgrade to the following minimum versions:

• Version 11.3.x: Upgrade to version 11.3.1 or later
• Version 11.2.x: Upgrade to version 11.2.3 or later
• Version 10.11.x: Upgrade to version 10.11.11 or later

For detailed patch information, refer to the Mattermost Security Updates page. The advisory tracking ID for this vulnerability is MMSA-2025-00568.

Workarounds

• Implement network-level restrictions to limit API access to trusted networks only
• Configure shorter session timeouts to reduce the window of opportunity for exploitation
• Manually invalidate user sessions immediately after removing them from teams
• Enable stricter API rate limiting on the channel search endpoint to slow enumeration attempts

# Configuration example - Reduce session timeout in Mattermost config.json
# Set shorter session length to minimize exploitation window
"ServiceSettings": {
    "SessionLengthWebInHours": 24,
    "SessionLengthMobileInHours": 720,
    "SessionLengthSSOInHours": 24,
    "SessionCacheInMinutes": 10
}