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Mattermost Server 11.4.x <= 11.4.3 / 11.5.x <= 11.5.1 Origin Validation Error (MMSA-2026-00636)

medium Nessus Plugin ID 316491

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Synopsis

The remote host is affected by an origin validation vulnerability.

Description

The version of Mattermost Server installed on the remote host is affected by a vulnerability as referenced in the MMSA-2026-00636 advisory.

 - 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. (CVE-2026-6339)

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 11.4.4, 11.5.2, 11.6.0 or later.

See Also

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

Plugin Details

Severity: Medium

ID: 316491

File Name: mattermost_server_MMSA-2026-00636.nasl

Version: 1.2

Type: Remote

Family: CGI abuses

Published: 5/22/2026

Updated: 5/25/2026

Supported Sensors: Nessus

Risk Information

VPR

Risk Factor: Low

Score: 2.2

CVSS v2

Risk Factor: Medium

Base Score: 4

Temporal Score: 3

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

CVSS Score Source: CVE-2026-6339

CVSS v3

Risk Factor: Medium

Base Score: 4.3

Temporal Score: 3.8

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

Temporal Vector: CVSS:3.0/E:U/RL:O/RC:C

Vulnerability Information

CPE: cpe:/a:mattermost:mattermost_server

Required KB Items: installed_sw/Mattermost Server

Exploit Ease: No known exploits are available

Patch Publication Date: 4/15/2026

Vulnerability Publication Date: 4/15/2026

Reference Information

CVE: CVE-2026-6339

IAVA: 2026-A-0492