Detections
Analytics

Mattermost Server 10.11.x < 10.11.22 / 11.7.x < 11.7.7 / 11.8.x < 11.8.4 Multiple Vulnerabilities (MMSA-2026-00685, MMSA-2026-00686, MMSA-2026-00687, MMSA-2026-00691)

high Nessus Plugin ID 338329

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Synopsis

The remote host is affected by multiple vulnerabilities.

Description

The version of Mattermost Server installed on the remote host is affected by multiple vulnerabilities as referenced in the MMSA-2026-00685, MMSA-2026-00686, MMSA-2026-00687, MMSA-2026-00691 advisories.

 - Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to validate BoardMember.Scheme* fields server-side on insert and archive-import paths which allows a board editor or non-guest team member to grant board admin to arbitrary users via POST /api/v2/boards/{boardID}/members and POST /api/v2/teams/{teamID}/archive/import. Mattermost Advisory ID: MMSA-2026-00685. (CVE-2026-9816)

 - Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to enforce PermissionManageBoardRoles on the channelId field of the batch endpoint, which allows an authenticated board editor to relink any board they can edit to an arbitrary channel via a crafted PATCH request. Mattermost Advisory ID: MMSA-2026-00686. (CVE-2026-9859)

 - Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fails to validate WebSocket command field types which allows an authenticated user to crash the plugin process and deny service to all Boards users via a custom_focalboard_SUBSCRIBE_TEAM message with a non-string teamId.
 Mattermost Advisory ID: MMSA-2026-00687. (CVE-2026-10080)

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 Server version 10.11.22, 11.7.7, 11.8.4, 11.9.0 or later.

See Also

https://mattermost.com/security-updates/

Plugin Details

Severity: High

ID: 338329

File Name: mattermost_server_MMSA-2026-00685_00686_00687_00691.nasl

Version: 1.1

Type: Remote

Family: CGI abuses

Published: 8/20/2026

Updated: 8/20/2026

Supported Sensors: Nessus

Risk Information

VPR

Risk Factor: Medium

Score: 4.5

Percentile: 57.58

CVSS v2

Risk Factor: High

Base Score: 8.7

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

CVSS Score Source: CVE-2026-9816

CVSS v3

Risk Factor: High

Base Score: 8.3

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

Vulnerability Information

CPE: cpe:/a:mattermost:mattermost_server

Required KB Items: installed_sw/Mattermost Server

Patch Publication Date: 7/17/2026

Vulnerability Publication Date: 8/17/2026

Reference Information

CVE: CVE-2026-10080, CVE-2026-10527, CVE-2026-9816, CVE-2026-9859