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

Severity: medium (CVSS 5.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-26 · updated 2026-06-17
Weakness: CWE-863

Description

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 products

Mattermost: from 10.11.0 before 10.11.11; from 11.2.0 before 11.2.3; from 11.3.0 before 11.3.2; from 11.4.0 before 11.4.1 — fixed in 10.11.11, 11.2.3, 11.3.2, 11.4.1

How to fix

Upgrade to a fixed version: Mattermost 10.11.11, 11.2.3, 11.3.2, 11.4.1. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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