CVE-2026-18795 Missing server side validation allowed user to quarantine a DM and GM post
An authenticated user can improperly quarantine direct or group messages via direct API calls due to missing server-side validation.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-18795?
CVE-2026-18795 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 16 Oct 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official 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
Weakness: CWE-862
Which versions are affected by CVE-2026-18795?
It affects 1 product.
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-18795?
No fix has been published for CVE-2026-18795 yet. Limit exposure of the affected software until one is available.
Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.
Is CVE-2026-18795 being exploited?
There is no known exploitation of CVE-2026-18795 in the wild and no public exploit code.
Other vulnerabilities in Mattermost Mattermost Server
- CVE-2025-12419 Account takeover on OAuth/OpenID-enabled servers
- CVE-2025-12421 Account Takeover via Code Exchange Endpoint
- CVE-2025-4981 Path Traversal Leading to RCE by Any Authenticated Mattermost User
- CVE-2025-20051 Arbitrary file read via block duplication in Mattermost Boards
- CVE-2025-25279 Arbitrary file read in Mattermost Boards via import & export board archive
Sources
Also known as GHSA-c5qm-9j4f-82mg.