CVE-2026-4274: 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…

PriorityP430medium5.4CVSS 3.1

AVNACLPRLUINSUCLILAN

EPSS

0.14%

3.8th percentile

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

13 ranges

Vendor	Product	Version range	Fixed in
github.com	mattermost_mattermost_server_v8	>= 10.11.0-rc1 < 10.11.11	10.11.11
github.com	mattermost_mattermost_server_v8	>= 11.2.0-rc1 < 11.2.3	11.2.3
github.com	mattermost_mattermost_server_v8	>= 11.3.0-rc1 < 11.3.2	11.3.2
github.com	mattermost_mattermost_server_v8	>= 11.4.0-rc1 < 11.4.1	11.4.1
github.com	mattermost_mattermost_server_v8	>= 8.0.0-20260105080200-d27a2195068d < 8.0.0-20260217110922-b7d4a1f1f59b	8.0.0-20260217110922-b7d4a1f1f59b
mattermost	mattermost	10.11.0 – 10.11.10	—
mattermost	mattermost	11.2.0 – 11.2.2	—
mattermost	mattermost	11.3.0 – 11.3.1	—
mattermost	mattermost	11.4.0 – 11.4.0	—
mattermost	mattermost_server	>= 10.11.0 < 10.11.11	10.11.11
mattermost	mattermost_server	>= 11.2.0 < 11.2.3	11.2.3
mattermost	mattermost_server	>= 11.3.0 < 11.3.2	11.3.2
mattermost	mattermost_server	>= 11.4.0 < 11.4.1	11.4.1

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