CVE-2025-20051: Mattermost Server Path Traversal Flaw

CVE-2025-20051 Overview

CVE-2025-20051 is a Path Traversal vulnerability affecting Mattermost Server that allows authenticated users to read arbitrary files from the system. The vulnerability exists due to improper input validation when patching and duplicating boards within the Mattermost Boards feature. An attacker with valid user credentials can craft a malicious block that, when duplicated, enables unauthorized access to sensitive files on the server filesystem.

Critical Impact

Authenticated attackers can read any file on the server filesystem, potentially exposing sensitive configuration files, credentials, private keys, and other confidential data stored on the Mattermost server.

Affected Products

• Mattermost Server versions 10.4.x up to and including 10.4.1
• Mattermost Server versions 10.3.x up to and including 10.3.2
• Mattermost Server versions 10.2.x up to and including 10.2.2
• Mattermost Server versions 9.11.x up to and including 9.11.7

Discovery Timeline

• 2025-02-24 - CVE-2025-20051 published to NVD
• 2025-08-18 - Last updated in NVD database

Technical Details for CVE-2025-20051

Vulnerability Analysis

This vulnerability is classified as CWE-22 (Path Traversal), which occurs when user-supplied input containing path traversal sequences is not properly sanitized before being used in file system operations. In the context of Mattermost Boards, the application fails to adequately validate input during board patching and duplication operations.

When a user creates or modifies a board block, the application processes file path references without proper sanitization. By including directory traversal sequences (such as ../) in the block data, an attacker can escape the intended directory structure and access files outside the application's designated storage area.

The network-based attack vector means this vulnerability can be exploited remotely by any authenticated user with access to the Boards feature. No user interaction is required beyond the attacker's own actions, and the confidentiality impact is significant as arbitrary file contents can be disclosed.

Root Cause

The root cause of CVE-2025-20051 is insufficient input validation in the Boards component when processing file path references during board patch and duplication operations. The application does not properly sanitize or validate user-controlled input that is subsequently used to construct file paths, allowing path traversal sequences to be processed and interpreted by the underlying filesystem operations.

Attack Vector

The attack is executed through the Mattermost Boards feature by exploiting the board duplication functionality. An authenticated attacker follows this attack pattern:

1. The attacker creates or modifies a board block containing specially crafted path traversal sequences in file reference fields
2. The attacker initiates a board duplication operation targeting this malicious block
3. The server processes the duplication request without properly validating the file path references
4. The path traversal sequences cause the application to read files outside the intended directory
5. The file contents are returned to the attacker as part of the duplicated board data

This attack requires only low-privilege authenticated access to the Mattermost instance with Boards functionality enabled. The vulnerability allows reading sensitive system files such as /etc/passwd, application configuration files containing database credentials, or private keys stored on the server.

Detection Methods for CVE-2025-20051

Indicators of Compromise

• Unusual board duplication requests containing path traversal patterns such as ../, ..%2f, or ..%252f in API requests
• Access logs showing repeated board patch or duplication operations targeting the same blocks
• Board blocks containing suspicious file path references that include parent directory navigation
• Unexpected file access patterns in server logs outside the Mattermost data directory

Detection Strategies

• Monitor Mattermost API endpoints related to board operations (/api/v2/boards/, /plugins/focalboard/) for path traversal patterns in request bodies
• Implement web application firewall (WAF) rules to detect and block common path traversal sequences in HTTP requests
• Enable detailed logging for Boards operations and review for anomalous file path references
• Deploy file integrity monitoring on sensitive system files to detect unauthorized read access

Monitoring Recommendations

• Configure SentinelOne to monitor for suspicious file read operations originating from the Mattermost server process
• Set up alerts for any Mattermost process attempting to read files outside its designated data directories
• Review Mattermost server logs for errors or warnings related to file operations in the Boards component
• Implement rate limiting on board duplication endpoints to slow potential exploitation attempts

How to Mitigate CVE-2025-20051

Immediate Actions Required

• Upgrade Mattermost Server to the latest patched version immediately
• For version 10.4.x, upgrade to version 10.4.2 or later
• For version 10.3.x, upgrade to version 10.3.3 or later
• For version 10.2.x, upgrade to version 10.2.3 or later
• For version 9.11.x, upgrade to version 9.11.8 or later
• Review server access logs for signs of exploitation prior to patching

Patch Information

Mattermost has released security patches addressing this vulnerability. Detailed patch information and updated software versions are available through the Mattermost Security Updates page. Organizations should prioritize upgrading to the latest stable release within their version branch to ensure complete protection against this and other security issues.

Workarounds

• If immediate patching is not possible, consider temporarily disabling the Boards plugin until the update can be applied
• Implement network-level restrictions to limit access to the Mattermost server to trusted users only
• Deploy a web application firewall with rules to detect and block path traversal attempts
• Restrict file system permissions for the Mattermost service account to minimize the impact of potential file read operations

# Disable Boards plugin via mmctl if immediate patching is not possible
mmctl plugin disable focalboard

# Verify the plugin is disabled
mmctl plugin list