---
id: "CVE-2026-18223"
kind: "vulnerability"
aliases: ["CVE-2026-18223"]
title: "Mattermost: invitation tokens redeemable by any account, exposing private channels"
severity: "medium"
cvss: 5.0
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["mattermost", "package", "self-hosted", "team-chat"]
not_affected_if: ["default-config"]
published_at: "2026-10-16T08:17:12+00:00"
first_seen_at: "2026-10-23T21:57:12+00:00"
updated_at: "2026-10-25T04:31:03+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-18223", "first_seen_at": "2026-10-23T21:57:12+00:00"}]
---

# Mattermost: invitation tokens redeemable by any account, exposing private channels

Mattermost fails to check that the signed-in user's email matches the invited recipient when redeeming team and guest invitation tokens. An authenticated user holding a forwarded or leaked invite link can join a team or reach private channels they were not invited to.

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

## Fix

- Upgrade Mattermost to a fixed release for your line: `11.10.1` or later for 11.10.x, `11.7.10` or later for 11.7.x, `11.9.2` or later for 11.9.x, `11.8.6` or later for 11.8.x (inferred from the affected ranges; confirm against MMSA-2026-00747)

## Timeline

- 2026-10-23T21:57:12+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-18223

## 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 invited recipient's email address when redeeming team and guest invitation tokens, which allows an authenticated attacker to join a team or gain unauthorized access to private channels via a forwarded or leaked invitation link.. Mattermost Advisory ID: MMSA-2026-00747

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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_