CVE-2026-4274: Insufficient authorization in shared channel membership sync grants team-level access instead of channel-level access
Mattermost versions 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to restrict team-level access when processing membership sync from a remote cluster, which allows a malicious remote cluster to grant a user access to an entire private team instead of only the shared channel via sending crafted membership sync messages that trigger team membership assignment. Mattermost Advisory ID: MMSA-2026-00574
Affected Software
Remediation
Information
Event History
Frequently Asked Questions
What is the severity of CVE-2026-4274?
CVE-2026-4274 is classified as a critical vulnerability due to insufficient authorization in shared channel membership sync.
How do I fix CVE-2026-4274?
To fix CVE-2026-4274, upgrade Mattermost to version 11.2.3, 10.11.11, 11.4.1, or 11.3.2 or later.
What are the affected versions in CVE-2026-4274?
Affected versions in CVE-2026-4274 include Mattermost versions 11.2.x up to 11.2.2, 10.11.x up to 10.11.10, 11.4.x up to 11.4.0, and 11.3.x up to 11.3.1.
What kind of access is mishandled in CVE-2026-4274?
CVE-2026-4274 mishandles access control, granting team-level access instead of the intended channel-level access during membership sync.
Are there any workarounds for CVE-2026-4274?
There are no documented workarounds for CVE-2026-4274; upgrading to a fixed version is strongly recommended.