Mattermost 11.5.x & 10.11.x Remote Cluster Membership Sync Bypass (CVE-2026-28759)
CVE-2026-28759 Published on May 18, 2026

Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels
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

Vendor Advisory NVD

Vulnerability Analysis

CVE-2026-28759 can be exploited with network access, and requires small amount of user privileges. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality, with no impact on integrity, and no impact on availability.

Attack Vector:
NETWORK
Attack Complexity:
LOW
Privileges Required:
LOW
User Interaction:
NONE
Scope:
UNCHANGED
Confidentiality Impact:
NONE
Integrity Impact:
LOW
Availability Impact:
NONE

Weakness Type

What is an AuthZ Vulnerability?

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.

CVE-2026-28759 has been classified to as an AuthZ vulnerability or weakness.


Products Associated with CVE-2026-28759

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MatterMost
 

Affected Versions

Mattermost:

Exploit Probability

EPSS
0.03%
Percentile
9.32%

EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.