CVE-2026-10556 in Mattermost
Summary
by MITRE • 09/14/2026
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
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 09/14/2026
The vulnerability identified in Mattermost versions 11.7.x through 11.9.x, specifically affecting releases up to 11.9.0, 11.8.4, and 10.11.22, represents a critical input validation failure within the Microsoft Graph webhook notification handling mechanism. This flaw stems from an insufficient check for null or empty values in incoming payload data sent via POST requests to public webhook endpoints. The affected component is responsible for processing calendar integration notifications provided by Microsoft Graph API services. When these payloads contain malformed entries where expected fields are null, the application fails to perform necessary defensive programming checks before attempting to process or serialize this data into internal structures. This lack of validation allows an unauthenticated attacker to exploit the logic error by crafting a specific HTTP POST request that includes null-valued parameters in critical sections of the webhook payload.
From a technical perspective, the core issue is classified as Improper Input Validation, which aligns with CWE-20 within the Common Weakness Enumeration standard. The vulnerability manifests when the application attempts to access properties or methods on objects derived from these unvalidated payloads without first verifying that they are not null. In many programming languages and runtime environments, such an operation triggers a NullPointerException or similar fatal exception. Because this processing occurs in a service thread dedicated to handling Microsoft Calendar plugin notifications, the resulting crash terminates the specific process responsible for managing calendar integrations. This is not merely a client-side display error but a server-side denial of condition that disrupts the availability of the integration layer itself.
The operational impact of this vulnerability is significant due to its unauthenticated nature and direct effect on service continuity. An attacker does not require valid credentials or prior access to the Mattermost instance to execute this attack vector. By simply sending a crafted POST request to the exposed webhook endpoint, an adversary can trigger the crash repeatedly or sustainably. The immediate consequence is the disruption of calendar integration services for all users on the affected Mattermost instance. Users relying on synchronized Microsoft Calendar events within their channels will experience service outages, missing updates, and potential data synchronization failures until the plugin process is manually restarted by system administrators. This constitutes a Denial of Service condition that impacts business continuity for organizations dependent on tight collaboration between messaging platforms and enterprise calendar systems.
This incident maps to several entries in the MITRE ATT&CK framework, primarily reflecting techniques associated with Resource Hijacking or Impact categories where availability is compromised. Specifically, it aligns with T1499 Endpoint Denial of Service if viewed as a local resource exhaustion, though more accurately it fits within broader service disruption patterns often seen in web application attacks targeting specific API endpoints. The attack vector leverages the public-facing nature of webhook URLs, which are designed to accept external data but must be rigorously sanitized against malformed inputs from potentially untrusted sources on the internet.
To mitigate this vulnerability and restore secure operations, organizations running affected versions should immediately upgrade Mattermost to a patched release that addresses MMSA-2026-00693. The patch introduces strict null-checking logic before any processing of webhook payload fields occurs, ensuring that malformed data is rejected or handled gracefully without causing process termination. In the interim, if upgrading is not immediately feasible, administrators should consider implementing network-level controls such as Web Application Firewalls to filter out requests with suspiciously structured payloads targeting the specific webhook endpoints associated with Microsoft Graph integrations. Additionally, restricting access to these webhook URLs via IP whitelisting can reduce the attack surface by limiting exposure to known Microsoft server ranges or internal proxies rather than leaving them open to the general internet. Regular auditing of input validation practices across all external-facing APIs is recommended to prevent similar flaws in other modules.