Mattermost Desktop 5.13.x < 5.13.6 / 6.x < 6.2.0 Multiple Vulnerabilities (MMSA-2026-00651 / MMSA-2026-00652)

high Nessus Plugin ID 321513

Synopsis

The remote host is affected by multiple vulnerabilities.

Description

The version of Mattermost Desktop installed on the remote host is affected by multiple vulnerabilities:

- Mattermost Desktop App versions <=6.1 5.5.13.0 fail to restrict the allow list of domains to which NTLM credentials were forwarded to in the Mattermost Desktop App which allows any user on a server without the image proxy enabled to intercept other users credentials via embedding an image that routes to an external web server. (CVE-2026-6517)

- Mattermost Desktop App versions <=6.1 5.5.13.0 fail to account for attempting to open extremely long URLs in the Mattermost Desktop App which allows a malicious server owner to crash the application via including a script to call window.open on a very large URL. (CVE-2026-8683)

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

Solution

Upgrade to Mattermost Desktop version 5.13.6, 6.2.0, or later.

Plugin Details

Severity: High

ID: 321513

File Name: mattermost_desktop_MMSA-2026-00651_MMSA-2026-00652.nasl

Version: 1.1

Type: Local

Agent: windows, macosx, unix

Family: Misc.

Published: 6/18/2026

Updated: 6/18/2026

Configuration: Enable thorough checks (optional)

Supported Sensors: Nessus Agent, Nessus

Risk Information

VPR

Risk Factor: Medium

Score: 5.2

CVSS v2

Risk Factor: High

Base Score: 7.8

Vector: CVSS2#AV:N/AC:L/Au:N/C:N/I:N/A:C

CVSS Score Source: CVE-2026-8683

CVSS v3

Risk Factor: High

Base Score: 7.7

Vector: CVSS:3.0/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N

CVSS Score Source: CVE-2026-6517

Vulnerability Information

CPE: cpe:/a:mattermost:mattermost_desktop

Patch Publication Date: 5/13/2026

Vulnerability Publication Date: 6/15/2026

Reference Information

CVE: CVE-2026-6517, CVE-2026-8683

Detections

Analytics