CVE-2026-28759: Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-05-18 · updated 2026-10-14
Weakness: CWE-863

Description

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate that a remote cluster has access to a channel before processing membership removal requests during shared channel membership sync, which allows a malicious remote cluster to remove any user from any channel, including private channels, via crafted membership sync messages targeting channels the remote cluster is not authorized to access. Mattermost Advisory ID: MMSA-2026-00576

Affected products

Mattermost: from 10.11.0 before 10.11.14; from 11.4.0 before 11.4.4; from 11.5.0 before 11.5.2 — fixed in 10.11.14, 11.4.4, 11.5.2

How to fix

Upgrade to a fixed version: Mattermost 10.11.14, 11.4.4, 11.5.2. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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