CVE-2026-4055 Insufficient permission validation on cross-team playbook run creation
Authenticated users can create playbook runs in unauthorized teams by manipulating team IDs, bypassing permission checks.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-4055?
CVE-2026-4055 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 21 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Mattermost versions 11.5.x <= 11.5.1 fail to validate team-level run_create permission against the target team when creating a playbook run which allows an authenticated team member to create runs in teams where they lack permission via specifying a different team ID in the run creation API request. Mattermost Advisory ID: MMSA-2026-00629
Weakness: CWE-863
Which versions are affected by CVE-2026-4055?
6 open source packages ship an affected version, including github.com/mattermost/mattermost/server/v8, github.com/mattermost/mattermost-server and github.com/mattermost/mattermost-server/v5. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
github.com/mattermost/mattermost/server/v8 Go | From 8.0.0-20260304132957-9f2616376582 | 8.0.0-20260320113102-f2b3d1c6a945 |
github.com/mattermost/mattermost/server/v8 Go | From 8.0.0-20260304132957-9f2616376582 | 8.0.0-20260320113102-f2b3d1c6a945 |
github.com/mattermost/mattermost/server/v8 Go | From 8.0.0-20260304132957-9f2616376582 | 8.0.0-20260320113102-f2b3d1c6a945 |
github.com/mattermost/mattermost-server Go | All versions | None |
github.com/mattermost/mattermost-server/v5 Go | All versions | None |
github.com/mattermost/mattermost-server/v6 Go | All versions | None |
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-4055?
Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20260320113102-f2b3d1c6a945 or later.
Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.
Is CVE-2026-4055 being exploited?
There is no known exploitation of CVE-2026-4055 in the wild and no public exploit code.
Other vulnerabilities in Mattermost Mattermost Server
- CVE-2025-12419 Account takeover on OAuth/OpenID-enabled servers
- CVE-2025-12421 Account Takeover via Code Exchange Endpoint
- CVE-2025-4981 Path Traversal Leading to RCE by Any Authenticated Mattermost User
- CVE-2025-20051 Arbitrary file read via block duplication in Mattermost Boards
- CVE-2025-25279 Arbitrary file read in Mattermost Boards via import & export board archive
Sources
Also known as GHSA-6cfr-wp44-6qmv, GO-2026-5173.