CVE-2026-28759CWE-863broken-access-control

CVE-2026-28759

Medium · published May 18, 2026

CVSS v3.1
4.3
EPSS
0%
Percentile
16.0
In the wild
Unconfirmed
What it is

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

The record
Technical detail
CVSS v3.1
4.3 · MEDIUM
Vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N
CVSS v4.0
Not supplied
EPSS
0.00254 · 16.0th percentile
Weakness
CWE-863 · Incorrect Authorization
Published
2026-05-18T12:16Z
Affected products (3)
ProductVersionsFixed in
mattermost/mattermost_server≥ 10.11.0, < 10.11.1410.11.14
mattermost/mattermost_server≥ 11.4.0, < 11.4.411.4.4
mattermost/mattermost_server≥ 11.5.0, < 11.5.211.5.2
References (1)
https://mattermost.com/security-updates · [email protected]
EPSS history
Timeline
  • 18 MAY 06:50Z
    Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels
    cvelistv5