CVE-2025-14350: Mattermost Server Information Disclosure

CVE-2025-14350 Overview

CVE-2025-14350 is a Missing Authorization vulnerability (CWE-862) affecting Mattermost Server that allows authenticated users to enumerate teams and their URL names through improper validation of team membership when processing channel mentions. By posting channel shortlinks and observing the channel_mentions property in the API response, attackers can determine the existence of teams they should not have visibility into.

Critical Impact

Authenticated attackers can discover hidden teams and their URL structures, potentially exposing organizational information and enabling further targeted attacks against specific team resources.

Affected Products

• Mattermost Server versions 11.1.x <= 11.1.2
• Mattermost Server versions 10.11.x <= 10.11.9
• Mattermost Server versions 11.2.x <= 11.2.1

Discovery Timeline

• 2026-02-16 - CVE-2025-14350 published to NVD
• 2026-02-18 - Last updated in NVD database

Technical Details for CVE-2025-14350

Vulnerability Analysis

This vulnerability stems from missing authorization checks in the channel mentions processing functionality of Mattermost Server. When a user posts a message containing a channel shortlink, the server processes the mention and returns information about the referenced channel through the channel_mentions property in the API response. The issue arises because the server fails to verify whether the authenticated user has legitimate membership in the team that owns the referenced channel before returning this information.

The information disclosure allows authenticated users to probe for the existence of teams and extract their URL naming conventions. While the vulnerability requires authentication to exploit, it breaks the expected isolation between teams and undermines the access control model that organizations rely upon to segment their communication channels.

Root Cause

The root cause is a missing authorization check (CWE-862) in the API endpoint that handles channel mention resolution. The server processes channel shortlinks and populates the channel_mentions response property without first validating that the requesting user has membership in the team associated with the mentioned channel. This breaks the principle of least privilege and allows information to leak across team boundaries.

Attack Vector

The attack is conducted over the network by authenticated users who can post messages containing channel shortlinks. The attacker crafts messages with shortlinks referencing channels in teams they do not belong to. By systematically probing different team URL patterns and analyzing the API responses, the attacker can enumerate which teams exist on the Mattermost instance and determine their URL structures. This reconnaissance information could be used to plan subsequent attacks or gain organizational intelligence.

The vulnerability is exploited through the standard Mattermost messaging workflow:

1. Attacker authenticates to the Mattermost instance with a valid user account
2. Attacker posts messages containing channel shortlinks with various team URL guesses
3. The API response's channel_mentions property reveals whether the referenced team exists
4. Attacker iterates through potential team names to build a map of existing teams

Detection Methods for CVE-2025-14350

Indicators of Compromise

• Unusual patterns of message posting containing channel shortlinks from a single user
• API requests to channels in teams where the requesting user has no membership
• High-frequency message creation followed by immediate deletion
• Anomalous channel_mentions property access patterns in API logs

Detection Strategies

• Monitor Mattermost application logs for users attempting to reference channels in teams they do not belong to
• Implement rate limiting on message creation to slow enumeration attempts
• Configure alerting for users posting messages with channel shortlinks to non-existent or unauthorized teams
• Review API access logs for patterns consistent with team enumeration behavior

Monitoring Recommendations

• Enable detailed logging for channel mention resolution in Mattermost Server
• Configure SIEM rules to detect repetitive channel shortlink patterns from single users
• Monitor for correlation between failed team access attempts and channel mention activity
• Establish baseline metrics for normal channel mention behavior to identify anomalies

How to Mitigate CVE-2025-14350

Immediate Actions Required

• Upgrade Mattermost Server to patched versions as soon as available
• Review recent API logs for signs of team enumeration activity
• Audit user accounts for suspicious messaging patterns involving channel shortlinks
• Consider restricting channel mention features until patches are applied

Patch Information

Mattermost has released security updates to address this vulnerability. Organizations should upgrade to versions newer than 11.1.2, 10.11.9, and 11.2.1 respectively for each affected branch. For detailed patch information, refer to the Mattermost Security Updates page. The Mattermost Advisory ID for this vulnerability is MMSA-2025-00563.

Workarounds

• Limit user registration and authentication to reduce the potential attacker pool
• Implement network segmentation to restrict Mattermost API access to trusted networks
• Deploy a Web Application Firewall (WAF) to monitor and filter suspicious channel mention patterns
• Consider temporarily disabling channel shortlink processing if the risk is deemed critical for your environment