CVE-2026-3433 Mattermost fails to scope role_updated websocket events to authorized team and channel members
An authenticated guest can observe private team permission changes via websocket events, leaking sensitive configuration data.
- 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-2026-3433?
CVE-2026-3433 is a medium-severity vulnerability in Mattermost Mattermost Server, published on 12 Jun 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 fail to restrict role_updated websocket event broadcasts to members of the affected team or channel which allows an authenticated attacker with guest-level access to observe permission scheme change notifications for private teams they are not a member of via the websocket connection.. Mattermost Advisory ID: MMSA-2026-00616
Weakness: CWE-200
Which versions are affected by CVE-2026-3433?
12 open source packages ship an affected version, including github.com/mattermost/mattermost-server, github.com/mattermost/mattermost-server/v5 and github.com/mattermost/mattermost-server/v6. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
github.com/mattermost/mattermost-server Go | From 10.11.0 | 10.11.17 |
github.com/mattermost/mattermost-server Go | From 10.11.0 | 10.11.17 |
github.com/mattermost/mattermost-server Go | From 10.11.0+incompatible | 10.11.17+incompatible, 11.5.5+incompatible, 11.6.1+incompatible |
github.com/mattermost/mattermost-server Go | From 11.5.0 | 11.5.5 |
github.com/mattermost/mattermost-server Go | From 11.5.0 | 11.5.5 |
github.com/mattermost/mattermost-server Go | From 11.6.0 | 11.6.1 |
6 more packages
github.com/mattermost/mattermost-server Go | From 11.6.0 | 11.6.1 |
github.com/mattermost/mattermost-server/v5 Go | All versions | None |
github.com/mattermost/mattermost-server/v6 Go | All versions | None |
github.com/mattermost/mattermost/server/v8 Go | From 8.0.0-20250731163400-5b955468ea1e | 8.0.0-20260504071740-9408b98025d7 |
github.com/mattermost/mattermost/server/v8 Go | From 8.0.0-20250731163400-5b955468ea1e | 8.0.0-20260504071740-9408b98025d7 |
github.com/mattermost/mattermost/server/v8 Go | From 8.0.0-20250731163400-5b955468ea1e | 8.0.0-20260504071740-9408b98025d7 |
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-3433?
Upgrade github.com/mattermost/mattermost-server to a fixed version for your release line: 10.11.17, 10.11.17+incompatible, 11.5.5+incompatible or 11.6.1+incompatible.
Is CVE-2026-3433 being exploited?
There is no known exploitation of CVE-2026-3433 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-rp4v-qc77-phm4, GO-2026-6286.