CVE-2026-8821 Mattermost - Broken Access Control

Mattermost 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 contains a broken access control caused by failure to validate channel member-management permission during playbook run creation, letting authenticated channel members add arbitrary users to restricted channels via the run owner field, exploit requires authentication.

CVSS base score 7.1 /10
High
CISA KEV
Not listed
Public exploit
None known
Authentication
Required
EPSS
0.29% chance of exploitation in 30 days
View the NVD record ↗

Details

Type
Broken Access Control
Impact
Broken Access Control
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N
Published
14 September 2026
Updated
15 October 2026
Weakness
CWE-862

Vendor fix

Update to versions later than 11.9.0, 11.8.4, 11.7.7, or 10.11.22 or the latest available version.

Patching closes this CVE. It does not tell you which of your systems run the affected version, or what else is exposed alongside them.

References

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