CVE-2026-10556 Unauthenticated webhook request with null notification entry could crash the Microsoft Calendar plugin.
An unauthenticated attacker can crash the Microsoft Calendar plugin by sending a crafted webhook request with null entries, denying calendar services.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-10556?
CVE-2026-10556 is a medium-severity vulnerability in Mattermost Mattermost Server, published on 14 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to validate null entries in Microsoft Graph webhook notification payloads, which allows an unauthenticated attacker to crash the Microsoft Calendar plugin process and deny calendar integration service to all users on the instance via a crafted {{POST}} request to the public webhook endpoint.. Mattermost Advisory ID: MMSA-2026-00693
Which versions are affected by CVE-2026-10556?
It affects 1 product.
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-10556?
Upgrade to a fixed version.
Is CVE-2026-10556 being exploited?
There is no known exploitation of CVE-2026-10556 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-56v5-m3m2-x7qh, MINI-78pp-q87v-w78r, MINI-gpxw-8c6h-v6c6.