CVE-2026-28759

EUVD-2026-30738

18.05.2026, 08:16

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

Provider	Type	Base Score	Atk. Vector	Atk. Complexity	Priv. Required	Vector
Mattermost	CNA	4.3 MEDIUM	NETWORK	LOW	LOW	CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N

Base Score

CVSS 3.x

EPSS Score

Percentile: 8%

Affected Products (NVD)

Vendor	Product	Version
mattermost	mattermost_server	10.11.0 ≤ 𝑥 < 10.11.14
mattermost	mattermost_server	11.4.0 ≤ 𝑥 < 11.4.4
mattermost	mattermost_server	11.5.0 ≤ 𝑥 < 11.5.2

𝑥

= Vulnerable software versions

Early Detection

Affected products identified ahead of NVD analysis through intelligence sources.

Vendor	Product	Version	Source
mattermost	mattermost	11.5.0 ≤ 𝑥 ≤ 11.5.1	CNA
mattermost	mattermost	10.11.0 ≤ 𝑥 ≤ 10.11.13	CNA
mattermost	mattermost	11.4.0 ≤ 𝑥 ≤ 11.4.3	CNA

Common Weakness Enumeration

CWE-863 - Incorrect Authorization
The software performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check. This allows attackers to bypass intended access restrictions.

References

https://mattermost.com/security-updates