Inconsistent Error Handling in Mattermost Leading to Channel Information Disclosure
CVE-2026-21386

4.3MEDIUM

Key Information:

Vendor

Mattermost
Status
Mattermost
Vendor
CVE Published:
16 March 2026

What is CVE-2026-21386?

The Mattermost platform exhibits inconsistent error handling when processing the /mute command. This flaw enables authenticated users to deduce the existence of private channels they do not have permissions to access. If different error messages are returned for non-existent channels versus private ones, it allows these users to exploit the information gained, potentially exposing sensitive data about the team's structure. Mitigation steps are advised to enhance security in upcoming updates.

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Affected Version(s)

Mattermost 11.3.0

Mattermost 11.2.0 <= 11.2.2

Mattermost 10.11.0 <= 10.11.10

References

https://mattermost.com/security-updates
vendor-advisory

CVSS V3.1

Score:
4.3
Severity:
MEDIUM
Confidentiality:
Low
Integrity:
None
Availability:
Low
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
Low
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

winfunc
JSON
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