CVE-2026-21386 Details
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
A vulnerability exists in Mattermost versions 11.3.x through 11.3.0, 11.2.x through 11.2.2, and 10.11.x through 10.11.10. The issue arises from the application's failure to provide uniform error responses when processing the /mute command. This inconsistency allows an authenticated team member to infer the existence of private channels they are not authorized to access, based on the differing error messages related to nonexistent channels versus private ones.
Users can upgrade to Mattermost versions 11.5.0, 11.4.0, or 11.3.1 to address this vulnerability.
Metrics
CVSS 4.0 Severity and Vector Strings:
No CVSS 4.0 data is available for this CVE.
CVSS 3.x Severity and Vector Strings:
No data available for CVSS Version 2.0 on this CVE.
CISA-ADP
Assessed Mar 16, 2026References to Advisories, Solutions, and Tools
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| URL | Source(s) | Tag(s) |
|---|---|---|
| https://mattermost.com/security-updates | [email protected] | Vendor Advisory |
Weakness Enumeration
| CWE-ID | CWE Name | Source |
|---|---|---|
| CWE-203 | Observable Discrepancy | [email protected] |
Affected Products
| Product | Versions |
|---|---|
| mattermost mattermost server | >= 10.11.0, < 10.11.11 >= 11.2.0, < 11.2.3 >= 11.3.0, < 11.3.1 |
CPE
Remediation
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Change History
4 change records found show changes
| Date | Action | Recorded By |
|---|---|---|
| Jun 17, 2026 | CVE Modified | [email protected] |
| Jun 17, 2026 | CVE Modified | CISA-ADP |
| Mar 18, 2026 | Initial Analysis | [email protected] |
| Mar 16, 2026 | New CVE Received | [email protected] |