Mattermost fails to use consistent error responses when handling the /mute command

CVE-2026-21386 → View on GitHub ↗ View on CVE.org ↗ View on NVD ↗

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

Basic information

Type
reviewed
Severity
medium
Advisory on GitHub
Open advisory ↗
Repository advisory
—
Source code
Browse source ↗
Published (advisory)
2026-03-16 15:30:46 UTC
Updated
2026-03-18 16:08:44 UTC
GitHub reviewed
2026-03-18 16:08:43 UTC
NVD published
2026-03-16

EPSS Score

Score Percentile
0.04% 10.94%

CVSS Scores

Base score Version Severity Vector
4.3 3.1 —
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:L)
A normal user session is enough; they don’t have to be admin.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:L)
Some sensitive info could get out, but not a total data dump.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:N)
Service keeps running; no real outage angle.

Identifiers

Type Value
GHSA GHSA-5mr9-crcg-8wh2 ↗
CVE CVE-2026-21386 ↗

CWEs

CWE id Name
CWE-203 Observable Discrepancy

Affected packages (5)

Vulnerable version ranges and first patched releases as published by GitHub.

Ecosystem Package Vulnerable range First patched Vulnerable functions
go github.com/mattermost/mattermost/server/v8 < 8.0.0-20260130144323-5bb5261c72fa 8.0.0-20260130144323-5bb5261c72fa —
go github.com/mattermost/mattermost-server < 5.3.2-0.20260130144323-5bb5261c72fa 5.3.2-0.20260130144323-5bb5261c72fa —
go github.com/mattermost/mattermost-server >= 10.11.0-rc1, < 10.11.11 10.11.11 —
go github.com/mattermost/mattermost-server >= 11.2.0-rc1, < 11.2.3 11.2.3 —
go github.com/mattermost/mattermost-server >= 11.3.0-rc1, < 11.3.1 11.3.1 —

References

cvelogic Threat Intelligence