Vulnerability · Published Mar 16, 2026 · Assigned by Mattermost
CVE-2026-21386Private channel enumeration via /mute slash command
Medium 4.3
In plain English
A medium-severity security flaw in Mattermost from Mattermost. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 9% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
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
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Go | github.com/mattermost/mattermost-server | 11.2.3 | GHSA-5mr9-crcg-8wh2 |
| Go | github.com/mattermost/mattermost-server | 5.3.2-0.20260130144323-5bb5261c72fa | GHSA-5mr9-crcg-8wh2 |
| Go | github.com/mattermost/mattermost-server | 10.11.11 | GHSA-5mr9-crcg-8wh2 |
| Go | github.com/mattermost/mattermost-server | 11.3.1 | GHSA-5mr9-crcg-8wh2 |
| Go | github.com/mattermost/mattermost-server | 10.11.11+incompatible, 11.2.3+incompatible, 11.3.1+incompatible | GO-2026-4744 |
| Go | github.com/mattermost/mattermost-server/v5 | no fix listed | GO-2026-4744 |
| Go | github.com/mattermost/mattermost-server/v6 | no fix listed | GO-2026-4744 |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260130144323-5bb5261c72fa | GO-2026-4744 |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260130144323-5bb5261c72fa | GHSA-5mr9-crcg-8wh2 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Mattermost | Mattermost | 11.3.0 – through 11.3.011.2.0 – through 11.2.210.11.0 – through 10.11.10 |
Weakness: CWE-203 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N
References
- MMSA-2026-00588 vendor-advisory