CVE-2026-95666 Unbounded post ID array in the bulk reactions endpoint allows denial of service
An authenticated user can cause denial of service by sending an unbounded post ID array to the bulk reactions endpoint, overloading the database.
- 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-2026-95666?
CVE-2026-95666 is a medium-severity denial of service vulnerability in Mattermost Mattermost Server, published on 22 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10, 11.10.x <= 11.10.1 fail to limit the length of the post ID array accepted by the bulk reactions endpoint which allows an authenticated user to cause excessive database load via a crafted request to {{POST /api/v4/posts/ids/reactions}}.. Mattermost Advisory ID: MMSA-2026-00771
Weakness: CWE-770
Which versions are affected by CVE-2026-95666?
It affects 1 product.
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-95666?
No fix has been published for CVE-2026-95666 yet. Limit exposure of the affected software until one is available.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-95666 being exploited?
There is no known exploitation of CVE-2026-95666 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-h2qv-vjc9-5g55.