Is CVE-2026-10556 being exploited?

No public exploit is currently tracked.

How severe is CVE-2026-10556?

CVE-2026-10556 has a CVSS v3 severity of MEDIUM (5.3).

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CVE-2026-10556

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