---
id: "CVE-2026-18795"
kind: "vulnerability"
aliases: ["CVE-2026-18795"]
title: "Mattermost: authenticated users can quarantine other users' direct message posts"
severity: "medium"
cvss: 6.4
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["chat", "mattermost", "package", "self-hosted"]
not_affected_if: ["default-config"]
published_at: "2026-10-16T07:17:12+00:00"
first_seen_at: "2026-10-23T21:57:12+00:00"
updated_at: "2026-10-25T03:59:46+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-18795", "first_seen_at": "2026-10-23T21:57:12+00:00"}]
---

# Mattermost: authenticated users can quarantine other users' direct message posts

Mattermost fails to check on the server that a post belongs to a direct or group message before letting a user quarantine it via the API. An authenticated user can therefore quarantine DM or GM posts they should not be able to act on.

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

## How to check

- Check the Mattermost server version with `mattermost version` or the System Console and compare against the affected ranges: 11.10.x up to 11.10.0, 11.9.x up to 11.9.1, 11.8.x up to 11.8.5, 11.7.x up to 11.7.9.

## Fix

- Upgrade Mattermost to a release outside the affected ranges (fixed versions unknown from the sources; see the Mattermost advisory MMSA-2026-00756).

## Timeline

- 2026-10-23T21:57:12+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-18795

## 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 enforce DM/GM check on server side which allows an authenticated user to quarantine a post belonging to DM or GM via direct API call.. Mattermost Advisory ID: MMSA-2026-00756

---

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