CVE-2026-21386

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

CVSS Metrics

• v3.1•MEDIUM•Score: 4.3CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N

EPSS Trends

Current EPSS score: 0.04%• Percentile: 13%

Techniques & Countermeasures

• CWE-203•Observable Discrepancy

  The product behaves differently or sends different responses under different circumstances in a way that is observable to an unauthorized actor.

Affected Systems

• github.com/mattermost•mattermost-server

  ≥ 11.3.0-rc1+incompatible, < 11.3.1+incompatible | < 5.3.2-0.20260130144323-5bb5261c72fa | ≥ 10.11.0-rc1, < 10.11.11 | ≥ 11.2.0-rc1, < 11.2.3 | ≥ 11.3.0-rc1, < 11.3.1

• github.com/mattermost/mattermost-server•v5

  all

• github.com/mattermost/mattermost-server•v6

  all

• github.com/mattermost/mattermost/server•v8

  < 8.0.0-20260130144323-5bb5261c72fa

• mattermost•mattermost

  11.3.0 | ≥ 11.2.0, ≤ 11.2.2 | ≥ 10.11.0, ≤ 10.11.10

• mattermost•mattermost_server

  ≥ 10.11.0, < 10.11.11 | ≥ 11.2.0, < 11.2.3 | ≥ 11.3.0, < 11.3.1

References (5)

• https://mattermost.com/security-updates
• https://nvd.nist.gov/vuln/detail/CVE-2026-21386
• https://github.com/mattermost/mattermost/commit/5bb5261c72faa476558a694c23581d24b734da41
• https://github.com/mattermost/mattermost
• https://github.com/advisories/GHSA-5mr9-crcg-8wh2