CVE intelligence and bounded remediation

CVE-2026-28759 — Mattermost Mattermost Server security vulnerability

Medium CVSS 4.3

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

Severity
Medium
CVSS
4.3 (3.1)
Published
2026-05-18
CISA KEV
Not currently listed
Ecosystem
software/application
Weaknesses
CWE-863

Affected products

  • mattermost / mattermost_server

Showing 1 representative product identities from 3 source matches. Confirm exact affected versions with the linked vendor and NVD evidence.

Matched remediation archetype

Authorization bypass, IDOR, and cross-tenant access

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Check exposure

  • Map object and action authorization checks across API, UI, batch, import/export, and background-job paths.
  • Identify tenant, ownership, role, and policy boundaries for affected resources and administrative operations.
  • Use synthetic fixtures to compare intended access matrices without accessing another user's real data.

Remediate safely

  • Enforce server-side authorization at each resource access and state transition using the authenticated principal and trusted tenant context.
  • Scope data queries by tenant and ownership; treat client-supplied identifiers, roles, and policy claims as untrusted.
  • Add deny-by-default policy tests for horizontal and vertical access across every affected transport.

Authoritative sources

Complete CVE record and remediation plan

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