CVE-2026-86348 MS Calendar plugin: unrecovered handler panics from malformed post-action requests could crash the plugin process
An authenticated user can crash the Mattermost Calendar plugin by sending a malformed post-action request, causing a denial of service.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-86348?
CVE-2026-86348 is a medium-severity denial of service vulnerability in Mattermost Mattermost Server, published on 14 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Mattermost versions <=11.9 11.0.9 11.4.8 11.7.7 10.22.11.0 fail to recover from handler panics, which allows an authenticated user to crash the plugin via a post-action request with an unexpected field type.. Mattermost Advisory ID: MMSA-2026-00701
Weakness: CWE-704
Which versions are affected by CVE-2026-86348?
It affects 1 product.
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-86348?
Upgrade to a fixed version.
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-2026-86348 being exploited?
There is no known exploitation of CVE-2026-86348 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-hw33-g6h4-vc78, MINI-3cq3-2w35-r43x, MINI-p4wq-vqcr-7mw5.