CVE-2026-9859

Medium

Description

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

SeverityMedium

CVSS6.5

Exploit probability—

Product

Mattermost Server

PublishedAug 17, 2026

First seenAug 17, 2026

Weakness typeCWE-863Incorrect authorization

How to fix

Update

mattermost mattermost_server10.11.22, 11.7.7, 11.8.4 or later

Vendor advisories

mattermost.com/security-updates

Affected products

Product	Affected versions	Fixed in
mattermost mattermost_server	10.11.0 ≤ v < 10.11.22, 11.7.0 ≤ v < 11.7.7, 11.8.0 ≤ v < 11.8.4	10.11.22, 11.7.7, 11.8.4

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