---
id: CVE-2026-16453
title: Mattermost SAML RelayState forgery allows unauthorized team membership
summary: >-
  Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1,
  11.8.x <= 11.8.5 fail to verify the integrity of the SAML RelayState POST
  parameter which allows an authenticated SAML user to join any team, including
  invite-on…
severity: high
cvss: 7.1
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N'
cvssSource: cna
cwe:
  - CWE-472
vendor: Mattermost
product: Mattermost
affected:
  - Mattermost >= 11.10.0 <= 11.10.0
  - Mattermost >= 11.7.0 <= 11.7.9
  - Mattermost >= 11.9.0 <= 11.9.1
  - Mattermost >= 11.8.0 <= 11.8.5
published: '2026-10-16'
updated: '2026-10-16'
sourceUpdated: '2026-10-16T07:08:48.627Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-16453'
references:
  - url: 'https://mattermost.com/security-updates'
    label: MMSA-2026-00727
tags:
  - cve.org
ingestedAt: '2026-10-16T08:02:23.121Z'
---

## Overview

Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to verify the integrity of the SAML RelayState POST parameter which allows an authenticated SAML user to join any team, including invite-only teams, via forging the team_id or invite_id fields in the RelayState.. Mattermost Advisory ID: MMSA-2026-00727

## Affected

- `Mattermost >= 11.10.0 <= 11.10.0`
- `Mattermost >= 11.7.0 <= 11.7.9`
- `Mattermost >= 11.9.0 <= 11.9.1`
- `Mattermost >= 11.8.0 <= 11.8.5`

## Remediation

Update Mattermost to versions 11.11.0, 11.10.1, 11.7.10, 11.9.2, 11.8.6 or higher.