CVE-2026-4274 — Insufficient authorization in shared channel membership sync grants team-level access instead of channel-level access

Description (CNA)

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 (CNA)

Vendor / product	Versions
Mattermost / Mattermost	affected 11.2.0 through 11.2.2; affected 10.11.0 through 10.11.10; affected 11.4.0 through 11.4.0; affected 11.3.0 through 11.3.1; unaffected 11.5.0; unaffected 11.2.3; unaffected 10.11.11; unaffected 11.4.1; unaffected 11.3.2

CWE

CWE-863

References

https://mattermost.com/security-updates

ADP enrichment

CISA-ADP

No English description supplied.

No references supplied.