---
id: CVE-2026-6339
title: >-
  Mattermost versions 11.5.x <= 11.5.1, 11.4.x <= 11.4.3 fail to validate the
  X-Requested-With header on the burn-on-read reveal endpoint which allows an
  authenticated channel member to force the reveal of a burn-on-read message
  without re…
summary: >-
  Mattermost versions 11.5.x <= 11.5.1, 11.4.x <= 11.4.3 fail to validate the
  X-Requested-With header on the burn-on-read reveal endpoint which allows an
  authenticated channel member to force the reveal of a burn-on-read message
  without re…
severity: medium
cvss: 4.3
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L'
cwe:
  - CWE-346
vendor: mattermost
product: mattermost_server
affected:
  - 'mattermost_server >= 11.4.0, < 11.4.4'
  - 'mattermost_server >= 11.5.0, < 11.5.2'
patched:
  - mattermost_server 11.5.2
published: '2026-05-18'
updated: '2026-10-14'
sourceUpdated: '2026-10-14T20:10:00.190'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-6339'
references:
  - url: 'https://mattermost.com/security-updates'
    label: responsibledisclosure@mattermost.com
tags:
  - nvd
epss: 0.00157
epssPercentile: 0.04349
ingestedAt: '2026-10-14T20:53:12.300Z'
---

## Overview

Mattermost versions 11.5.x <= 11.5.1, 11.4.x <= 11.4.3 fail to validate the X-Requested-With header on the burn-on-read reveal endpoint which allows an authenticated channel member to force the reveal of a burn-on-read message without recipient consent via a crafted Markdown image tag.. Mattermost Advisory ID: MMSA-2026-00636

## Affected

- `mattermost_server >= 11.4.0, < 11.4.4`
- `mattermost_server >= 11.5.0, < 11.5.2`

## Remediation

Upgrade past the affected range:

- `mattermost_server 11.5.2`