CVE-2026-4274 PUBLISHED

Insufficient authorization in shared channel membership sync grants team-level access instead of channel-level access

Assigner: Mattermost
Reserved: 16.03.2026 Published: 26.03.2026 Updated: 26.03.2026

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

Metrics

CVSS 3.1

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
CVSS Score: 5.4

Attack Vector	Network	Scope	Unchanged
Attack Complexity	Low	Confidentiality Impact	Low
Privileges Required	Low	Integrity Impact	Low
User Interaction	None	Availability Impact	None

CVSS 3.1

Product Status

Vendor	Mattermost
Product	Mattermost
Versions	Default: unaffected affected from 11.2.0 to 11.2.2 (incl.) affected from 10.11.0 to 10.11.10 (incl.) affected from 11.4.0 to 11.4.0 (incl.) affected from 11.3.0 to 11.3.1 (incl.) Version 11.5.0 is unaffected Version 11.2.3 is unaffected Version 10.11.11 is unaffected Version 11.4.1 is unaffected Version 11.3.2 is unaffected

Solutions

Update Mattermost to versions 11.5.0, 11.2.3, 10.11.11, 11.4.1, 11.3.2 or higher.

Credits

daw10 finder

References

MMSA-2026-00574

Problem Types

CWE-863: Incorrect Authorization CWE