CVE-2026-9859

What it is

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

At a glance

Exploitability

Can be exploited over the network with a low-privilege account. High risk of data exposure.

Exploit odds

0.3% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2026-08-17 22:17 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-08-19 13:28 UTC Last updatedMost recent change to the record

Details

Published

2026-08-17 22:17 UTC

Last modified

2026-08-19 13:28 UTC

Exploit probability

0.3% · EPSS, next 30 days

Weakness

CWE-863 · Incorrect authorization

Record status

Analyzed

Affected

• Mattermost Server < 10.11.22
• Mattermost Server < 11.7.7
• Mattermost Server < 11.8.4

Attack profile CVSS 3.1 · 6.5

Attack vector

Network

Complexity

Low

Privileges needed

Low

User interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

None

Availability

None