Mattermost <=11.9.0 Null Payload Crash in MS Graph Webhook CVE-2026-10556
CVE-2026-10556 Published on September 14, 2026
Unauthenticated webhook request with null notification entry could crash the Microsoft Calendar plugin.
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
Vulnerability Analysis
CVE-2026-10556 is exploitable with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity, and a small impact on availability.
Weakness Type
Improper Check for Unusual or Exceptional Conditions
The software does not check or incorrectly checks for unusual or exceptional conditions that are not expected to occur frequently during day to day operation of the software.
Products Associated with CVE-2026-10556
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Affected Versions
Mattermost:- Version 11.9.0, <= 11.9.0 is affected.
- Version 11.8.0, <= 11.8.4 is affected.
- Version 11.7.0, <= 11.7.7 is affected.
- Version 10.11.0, <= 10.11.22 is affected.
- Version 11.10.0 is unaffected.
- Version 11.9.1 is unaffected.
- Version 11.8.5 is unaffected.
- Version 11.7.8 is unaffected.
- Version 10.11.23 is unaffected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.