Detections
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Mattermost Server 11.5.x < 11.5.2 Missing Authorization (MMSA-2026-00645)

medium Nessus Plugin ID 316490

Synopsis

The remote host is affected by a missing authorization vulnerability.

Description

The version of Mattermost Server installed on the remote host is affected by a vulnerability as referenced in the MMSA-2026-00645 advisory.

 - Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to the post rewrite endpoint. Mattermost Advisory ID: MMSA-2026-00645. (CVE-2026-5163)

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

Solution

Upgrade to Mattermost Server version 11.5.2, 11.6.0 or later.

See Also

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

Plugin Details

Severity: Medium

ID: 316490

File Name: mattermost_server_MMSA-2026-00645.nasl

Version: 1.1

Type: Remote

Family: CGI abuses

Published: 5/22/2026

Updated: 5/22/2026

Supported Sensors: Nessus

Risk Information

VPR

Risk Factor: Medium

Score: 4.4

CVSS v2

Risk Factor: Medium

Base Score: 6.8

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

CVSS Score Source: CVE-2026-5163

CVSS v3

Risk Factor: Medium

Base Score: 6.5

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

Vulnerability Information

CPE: cpe:/a:mattermost:mattermost_server

Required KB Items: installed_sw/Mattermost Server

Patch Publication Date: 4/15/2026

Vulnerability Publication Date: 4/15/2026

Reference Information

CVE: CVE-2026-5163

IAVA: 2026-A-0492