---
id: "CVE-2026-16453"
kind: "vulnerability"
aliases: ["CVE-2026-16453"]
title: "Mattermost: forged SAML RelayState lets users join invite-only teams"
severity: "high"
cvss: 7.1
epss: null
exploited: false
status: "open"
category: "package"
products: [{"name": "mattermost", "fixed": ["11.7.10", "11.8.6", "11.9.2", "11.10.1", "11.11.0"], "affected": ">= 11.7.0, <= 11.7.9; >= 11.8.0, <= 11.8.5; >= 11.9.0, <= 11.9.1; 11.10.0", "ecosystem": "vendor"}]
ecosystems: []
tags: ["mattermost", "package", "saml", "self-hosted", "team-access"]
not_affected_if: ["no-saml", "patched-version", "saml-disabled"]
published_at: "2026-10-16T08:17:12+00:00"
first_seen_at: "2026-10-23T21:57:12+00:00"
updated_at: "2026-10-25T03:48:55+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-16453", "first_seen_at": "2026-10-23T21:57:12+00:00"}, {"source": "analyst", "url": null, "first_seen_at": "2026-10-25T03:48:55+00:00"}]
---

# Mattermost: forged SAML RelayState lets users join invite-only teams

Mattermost fails to verify the integrity of the SAML RelayState POST parameter. An authenticated SAML user can forge the team_id or invite_id fields to join any team, including invite-only teams.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- vendor:mattermost affected >= 11.7.0, <= 11.7.9; >= 11.8.0, <= 11.8.5; >= 11.9.0, <= 11.9.1; 11.10.0 fixed 11.7.10, 11.8.6, 11.9.2, 11.10.1, 11.11.0

## How to check

Check Mattermost version (System Console > About, or 'mmctl version' / server --version). Affected if running 11.7.0-11.7.9, 11.8.0-11.8.5, 11.9.0-11.9.1 or 11.10.0 AND SAML login is enabled (System Console > Authentication > SAML 2.0). Check team membership audit for unexpected joins of invite-only teams by SAML users.

## Fix

Upgrade to 11.11.0, 11.10.1, 11.9.2, 11.8.6 or 11.7.10 (or later on your release line). Fix signs and verifies SAML RelayState with HMAC-SHA256.

## Timeline

- 2026-10-23T21:57:12+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-16453
- 2026-10-25T03:48:55+00:00 analyst — Fixed versions from Mattermost CNA CVE record (mattermost.com security-updates page is JS-rendered, not readable); fix PR #37837 adds HMAC-SHA256 RelayState signing. No exploitation or PoC found; no social search done (low-profile B). Not marked verified: vendor advisory page itself not read.

## Documents

- [advisory] CVE-2026-16453 (CNA: Mattermost, MMSA-2026-00727) https://www.cve.org/CVERecord?id=CVE-2026-16453
- [fix] MM-69889 Improve handling of RelayState in SAML flow (HMAC-SHA256 signing) https://github.com/mattermost/mattermost/pull/37837

## From the sources

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

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