---
id: "CVE-2026-18551"
kind: "vulnerability"
aliases: ["CVE-2026-18551"]
title: "Mattermost: guest accounts can gain team or channel admin rights"
severity: "medium"
cvss: 6.5
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["guest-accounts", "mattermost", "package", "self-hosted"]
not_affected_if: ["fixed-version", "no-guest-accounts"]
published_at: "2026-10-16T07:17:12+00:00"
first_seen_at: "2026-10-23T21:57:12+00:00"
updated_at: "2026-10-25T03:58:01+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-18551", "first_seen_at": "2026-10-23T21:57:12+00:00"}]
---

# Mattermost: guest accounts can gain team or channel admin rights

Mattermost fails to validate combined guest and admin scheme roles, so an authenticated guest user can send a crafted PUT request to the member roles API. That lets the guest escalate to team or channel admin.

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

## How to check

- Check the running Mattermost server version with `mattermost version` or the System Console, and compare against the affected ranges: 11.7.x up to 11.7.9, 11.8.x up to 11.8.5, 11.9.x up to 11.9.1, and 11.10.0.
- Review whether guest accounts exist on the server in the System Console under Users, filtered by the guest role.

## Fix

- Upgrade Mattermost to a fixed release listed in advisory MMSA-2026-00752 for your release line (unknown exact fixed versions in the sources).
- Until patched, review team and channel admin assignments for guest accounts and remove any elevated roles.

## Timeline

- 2026-10-23T21:57:12+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-18551

## 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 validate the combination of guest and admin scheme roles which allows an authenticated attacker to escalate a guest account to team or channel admin via a crafted PUT request to the member roles API endpoint.. Mattermost Advisory ID: MMSA-2026-00752

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_