CVE-2026-18795: Missing server side validation allowed user to quarantine a DM and GM post

Severity: medium (CVSS 6.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-16 · updated 2026-10-29
Weakness: CWE-862

Description

Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to enforce DM/GM check on server side which allows an authenticated user to quarantine a post belonging to DM or GM via direct API call.. Mattermost Advisory ID: MMSA-2026-00756

Affected products

Mattermost: from 11.7.0 before 11.7.10; from 11.8.0 before 11.8.6; from 11.9.0 before 11.9.2; 11.10.0 (and their updates) — fixed in 11.7.10, 11.8.6, 11.9.2
Mattermost: from 11.10.0 through 11.10.0; from 11.7.0 through 11.7.9; from 11.9.0 through 11.9.1; from 11.8.0 through 11.8.5

How to fix

Upgrade to a fixed version: Mattermost 11.7.10, 11.8.6, 11.9.2. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

Get alerted when CVE-2026-18795 or the next vulnerability affects your stack