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Mattermost CVE-2026-10556

| EUVDEUVD-2026-77379 MEDIUM
Improper Check for Unusual or Exceptional Conditions (CWE-754)
2026-09-14 Mattermost GHSA-56v5-m3m2-x7qh
Denial Of Service Microsoft Mattermost
5.3
CVSS 3.1 · Vendor: Mattermost
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Severity by source

Vendor (Mattermost) PRIMARY
5.3 MEDIUM
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
vuln.today AI
5.3 MEDIUM

Public unauthenticated endpoint, no credentials needed; only availability impacted and only the plugin subprocess crashes, not the full server.

3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
4.0 AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

Primary rating from Vendor (Mattermost).

CVSS VectorVendor: Mattermost

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
Low

Lifecycle Timeline

2
Analysis Generated
Sep 15, 2026 - 07:11 vuln.today
CVE Published
Sep 14, 2026 - 10:46 cve.org
MEDIUM 5.3

DescriptionCVE.org

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

AnalysisAI

The Microsoft Calendar plugin in Mattermost fails to validate null entries in incoming Microsoft Graph webhook payloads, enabling any unauthenticated remote attacker to crash the plugin process by sending a single crafted POST request to the public webhook endpoint. Affected are Mattermost versions 11.9.0, 11.8.x through 11.8.4, 11.7.x through 11.7.7, and 10.11.x through 10.11.22. …

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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata

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Delivery
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Vulnerability AssessmentAI

Exploitation Exploitation requires that the Mattermost Microsoft Calendar plugin is installed and enabled on the target instance, which exposes a public unauthenticated HTTP webhook endpoint for receiving Microsoft Graph notifications. … Additional conditions and limiting factors are described in the full assessment.
Risk Assessment The CVSS 5.3 medium score is consistent with the actual impact: the vulnerability is trivially reachable (AV:N/AC:L/PR:N/UI:N) but the blast radius is limited to the calendar plugin subprocess rather than the core Mattermost server, hence A:L. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in.
Exploit Scenario Full exploit scenario with step-by-step reproduction available after sign-in.
Remediation Upgrade Mattermost to a version beyond the affected ranges - specifically a release newer than 11.9.0, 11.8.4, 11.7.7, or 10.11.22 as applicable to your branch. … Detailed patch versions, workarounds, and compensating controls in full report.
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CVE-2026-10556 vulnerability details – vuln.today
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