CVE-2026-21386 Private channel enumeration via /mute slash command
Authenticated users can enumerate private channels via inconsistent error messages in the /mute command, exposing unauthorized channel existence.
- 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-2026-21386?
CVE-2026-21386 is a medium-severity vulnerability in Mattermost Mattermost Server, published on 16 Mar 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to use consistent error responses when handling the /mute command which allows an authenticated team member to enumerate private channels they are not authorized to know about via differing error messages for nonexistent versus private channels. Mattermost Advisory ID: MMSA-2026-00588
Weakness: CWE-203
Which versions are affected by CVE-2026-21386?
14 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 | All versions | 5.3.2-0.20260130144323-5bb5261c72fa |
github.com/mattermost/mattermost-server Go | All versions | 5.3.2-0.20260130144323-5bb5261c72fa |
github.com/mattermost/mattermost-server Go | From 10.11.0-rc1 | 10.11.11 |
github.com/mattermost/mattermost-server Go | From 10.11.0-rc1 | 10.11.11 |
github.com/mattermost/mattermost-server Go | From 10.11.0-rc1+incompatible | 10.11.11+incompatible, 11.2.3+incompatible, 11.3.1+incompatible |
github.com/mattermost/mattermost-server Go | From 11.2.0-rc1 | 11.2.3 |
8 more packages
github.com/mattermost/mattermost-server Go | From 11.2.0-rc1 | 11.2.3 |
github.com/mattermost/mattermost-server Go | From 11.3.0-rc1 | 11.3.1 |
github.com/mattermost/mattermost-server Go | From 11.3.0-rc1 | 11.3.1 |
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 | All versions | 8.0.0-20260130144323-5bb5261c72fa |
github.com/mattermost/mattermost/server/v8 Go | All versions | 8.0.0-20260130144323-5bb5261c72fa |
github.com/mattermost/mattermost/server/v8 Go | All versions | 8.0.0-20260130144323-5bb5261c72fa |
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-21386?
Upgrade github.com/mattermost/mattermost-server to a fixed version for your release line: 5.3.2-0.20260130144323-5bb5261c72fa, 10.11.11, 10.11.11+incompatible or 11.2.3+incompatible.
Is CVE-2026-21386 being exploited?
There is no known exploitation of CVE-2026-21386 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-5mr9-crcg-8wh2, GO-2026-4744.