CVE-2025-13870: Mattermost Server Auth Bypass Vulnerability

CVE-2025-13870 Overview

CVE-2025-13870 is a medium-severity authorization bypass vulnerability affecting Mattermost Server. The vulnerability exists in the Boards feature where the application fails to properly validate user permissions when accessing files and subscribing to blocks. This broken access control flaw allows an authenticated user to access board files and subscribe to blocks from boards they do not have legitimate access to, potentially exposing sensitive information across organizational boundaries.

Critical Impact

Authenticated users can bypass authorization controls to access files and subscribe to blocks from boards they should not have access to, leading to unauthorized information disclosure.

Affected Products

• Mattermost Server versions 10.11.x through 10.11.4
• Mattermost Server versions 10.5.x through 10.5.12
• Mattermost Mattermost Server (all affected versions with Boards feature enabled)

Discovery Timeline

• 2025-12-02 - CVE-2025-13870 published to NVD
• 2025-12-03 - Last updated in NVD database

Technical Details for CVE-2025-13870

Vulnerability Analysis

This vulnerability is classified under CWE-306 (Missing Authentication for Critical Function), though it more accurately represents a broken access control issue where authentication exists but authorization validation is insufficient. The CVSS 3.1 score of 4.3 (Medium) reflects the network-accessible attack vector with low attack complexity, requiring low privileges with no user interaction needed.

The CVSS vector CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N indicates:

• Attack Vector (AV:N): Exploitable over the network
• Attack Complexity (AC:L): No specialized conditions required
• Privileges Required (PR:L): Attacker must have valid authentication credentials
• User Interaction (UI:N): No victim interaction needed
• Scope (S:U): Impact limited to the vulnerable component
• Confidentiality Impact (C:L): Limited information disclosure
• Integrity Impact (I:N): No modification capability
• Availability Impact (A:N): No service disruption

The Exploit Prediction Scoring System (EPSS) assigns this vulnerability a probability of 0.03% with a percentile ranking of 7.865, indicating relatively low likelihood of exploitation in the wild.

Root Cause

The root cause lies in missing or insufficient authorization checks within the Boards component of Mattermost Server. When processing requests to access board files or subscribe to block updates, the application validates that the user is authenticated but fails to verify whether the authenticated user has the necessary permissions to access the specific board resources being requested. This represents a classic broken access control pattern where authentication and authorization are conflated.

Attack Vector

An attacker with valid Mattermost credentials can exploit this vulnerability through network-based requests to the Boards API endpoints. The attack does not require any special privileges beyond basic authenticated user access.

The exploitation process involves:

1. The attacker authenticates to the Mattermost server with any valid user account
2. The attacker identifies or enumerates board IDs, file IDs, or block IDs belonging to boards they don't have access to
3. The attacker crafts API requests targeting these resources
4. Due to missing authorization validation, the server processes these requests and returns the requested data or subscribes the attacker to block updates

The vulnerability allows attackers to access files attached to restricted boards and receive real-time updates when blocks are modified on boards they should not be able to view.

Detection Methods for CVE-2025-13870

Indicators of Compromise

• Unusual API requests to Boards endpoints from users who are not members of the targeted boards
• Access logs showing file download requests for board attachments by non-board members
• Subscription activity to blocks from users without corresponding board membership
• Cross-board access patterns that violate expected organizational boundaries

Detection Strategies

Security teams should implement monitoring for anomalous access patterns in Mattermost Boards. This includes:

1. API Request Analysis: Monitor API calls to board file access and block subscription endpoints. Correlate the requesting user ID with board membership lists to identify unauthorized access attempts.

2. Access Log Review: Examine Mattermost server access logs for patterns where users successfully access board resources without appropriate membership or permissions.

3. User Behavior Analytics: Implement UBA rules to detect users accessing an unusually high number of boards or boards outside their typical interaction patterns.

Monitoring Recommendations

Organizations should enable comprehensive logging on their Mattermost deployments and forward logs to a centralized SIEM solution. Configure alerts for:

• Successful file access events where the user is not a board member
• Block subscription events for non-member users
• High-volume board enumeration attempts
• Failed and successful authorization bypass patterns

SentinelOne Singularity XDR can provide endpoint-level visibility into application behavior and network communications that may indicate exploitation attempts against Mattermost infrastructure.

How to Mitigate CVE-2025-13870

Immediate Actions Required

• Update Mattermost Server to a patched version immediately (versions newer than 10.11.4 for the 10.11.x branch or newer than 10.5.12 for the 10.5.x branch)
• Audit recent board access logs to identify any unauthorized access that may have occurred
• Review board permissions and membership to ensure proper access controls are configured
• Consider temporarily disabling the Boards feature if updates cannot be applied immediately

Patch Information

Mattermost has released security updates to address this vulnerability. Organizations should upgrade to the latest patched versions available through the official Mattermost distribution channels. Detailed patch information is available at the vendor security advisory: https://mattermost.com/security-updates

The patched versions implement proper authorization validation that verifies user board membership and permissions before allowing file access or block subscriptions.

Workarounds

If immediate patching is not feasible, organizations can implement the following temporary mitigations:

1. Restrict Boards Feature: Disable the Boards feature entirely through Mattermost system console if it is not business-critical
2. Network Segmentation: Limit network access to the Mattermost server to trusted network segments
3. Enhanced Monitoring: Implement strict monitoring of Boards API endpoints to detect and alert on potential exploitation
4. User Access Review: Audit and minimize the number of authenticated users with access to the Mattermost instance

# Review Mattermost configuration for Boards feature
# Location: /opt/mattermost/config/config.json
# Disable ProductSettings.EnableBoardsProduct if immediate patching is not possible

# Check current Mattermost version
/opt/mattermost/bin/mattermost version

# Monitor access logs for suspicious board access
grep -i "boards" /opt/mattermost/logs/mattermost.log | grep -v "authorized"