CVE-2026-28759 Mattermost does not verify remote cluster channel access when processing shared channel membership removals
A malicious remote cluster can remove users from arbitrary channels, including private ones, due to insufficient authorization checks during membership synchronization.
- 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-2026-28759?
CVE-2026-28759 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 18 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, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate that a remote cluster has access to a channel before processing membership removal requests during shared channel membership sync, which allows a malicious remote cluster to remove any user from any channel, including private channels, via crafted membership sync messages targeting channels the remote cluster is not authorized to access. Mattermost Advisory ID: MMSA-2026-00576
Weakness: CWE-863
Which versions are affected by CVE-2026-28759?
14 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 | All versions | 8.0.0-20260216150504-8738f8c4b3d4 |
github.com/mattermost/mattermost/server/v8 Go | All versions | 8.0.0-20260216150504-8738f8c4b3d4 |
github.com/mattermost/mattermost/server/v8 Go | All versions | 8.0.0-20260216150504-8738f8c4b3d4 |
github.com/mattermost/mattermost/server/v8 Go | From 10.11.0 | 10.11.14 |
github.com/mattermost/mattermost/server/v8 Go | From 10.11.0 | 10.11.14 |
github.com/mattermost/mattermost/server/v8 Go | From 11.4.0 | 11.4.4 |
8 more packages
github.com/mattermost/mattermost/server/v8 Go | From 11.4.0 | 11.4.4 |
github.com/mattermost/mattermost/server/v8 Go | From 11.5.0 | 11.5.2 |
github.com/mattermost/mattermost/server/v8 Go | From 11.5.0 | 11.5.2 |
github.com/mattermost/mattermost-server Go | All versions | 5.3.2-0.20260216150504-8738f8c4b3d4 |
github.com/mattermost/mattermost-server Go | All versions | 5.3.2-0.20260216150504-8738f8c4b3d4 |
github.com/mattermost/mattermost-server Go | From 10.11.0+incompatible | 10.11.14+incompatible, 11.4.4+incompatible, 11.5.2+incompatible |
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-2026-28759?
Upgrade github.com/mattermost/mattermost/server/v8 to a fixed version for your release line: 8.0.0-20260216150504-8738f8c4b3d4, 10.11.14, 11.4.4 or 11.5.2.
Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.
Is CVE-2026-28759 being exploited?
There is no known exploitation of CVE-2026-28759 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-8h9w-w78c-vvr3, GO-2026-5256.