CVE-2026-21386
March 16, 2026
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
github.com/mattermost/mattermost/server/v8 (GO):
Affected version(s) >=v8.0.0-20230429093631-aac4f2337933 <v8.0.0-20260130144323-5bb5261c72faFix Suggestion:
Update to version v8.0.0-20260130144323-5bb5261c72fagithub.com/mattermost/mattermost (GO):
Affected version(s) >=v11.2.0 <v11.2.3Fix Suggestion:
Update to version v11.2.3github.com/mattermost/mattermost (GO):
Affected version(s) =v11.3.0 <v11.3.1Fix Suggestion:
Update to version v11.3.1github.com/mattermost/mattermost-server (GO):
Affected version(s) >=v10.11.0-rc1 <v10.11.11Fix Suggestion:
Update to version v10.11.11github.com/mattermost/mattermost (GO):
Affected version(s) >=v10.11.0 <v10.11.11Fix Suggestion:
Update to version v10.11.11github.com/mattermost/mattermost-server (GO):
Affected version(s) >=v11.2.0-rc1 <v11.2.3Fix Suggestion:
Update to version v11.2.3github.com/mattermost/mattermost-server (GO):
Affected version(s) >=v11.3.0-rc1 <v11.3.1Fix Suggestion:
Update to version v11.3.1Related Resources (5)
Do you need more information?
Contact UsCVSS v4
Base Score:
5.3
Attack Vector
NETWORK
Attack Complexity
LOW
Attack Requirements
NONE
Privileges Required
LOW
User Interaction
NONE
Vulnerable System Confidentiality
LOW
Vulnerable System Integrity
NONE
Vulnerable System Availability
NONE
Subsequent System Confidentiality
NONE
Subsequent System Integrity
NONE
Subsequent System Availability
NONE
CVSS v3
Base Score:
4.3
Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
LOW
User Interaction
NONE
Scope
UNCHANGED
Confidentiality
LOW
Integrity
NONE
Availability
NONE
Weakness Type (CWE)
Observable Discrepancy
EPSS
Base Score:
0.03
