Permission Management Flaw in Mattermost Platform
CVE-2026-9859

6.5MEDIUM

Key Information:

Vendor

Mattermost
Status
Mattermost
Vendor
CVE Published:
17 August 2026

What is CVE-2026-9859?

The Mattermost platform suffers from a vulnerability due to improper enforcement of PermissionManageBoardRoles on the channelId field in the batch endpoint. This flaw allows authenticated board editors to exploit the system by relinking any board they have editing rights on to any arbitrary channel, facilitated through a maliciously crafted PATCH request. Affected versions include Mattermost 11.7.x up to 11.7.6, 10.11.x up to 10.11.21, and 11.8.x up to 11.8.3. System administrators are encouraged to review the Mattermost security advisory for more information and to implement necessary patches.

Affected Version(s)

Mattermost 11.7.0 <= 11.7.6

Mattermost 10.11.0 <= 10.11.21

Mattermost 11.8.0 <= 11.8.3

References

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

CVSS V3.1

Score:
6.5
Severity:
MEDIUM
Confidentiality:
High
Integrity:
None
Availability:
High
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
Low
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

kauasilvab14fb3
.