Unauthenticated webhook request with null notification entry could crash the Microsoft Calendar plugin.

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-10556
StatePUBLISHED
AssignerMattermost
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-09-14 11:17:02 UTC
Updated2026-10-07 16:25:31 UTC
DescriptionMattermost 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

Risk And Classification

Primary CVSS: v3.1 5.3 MEDIUM from [email protected]

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS: 0.002480000 probability, percentile 0.162970000 (date 2026-09-16)

Problem Types: CWE-754 | CWE-476 | CWE-754 CWE-754: Improper Check for Unusual or Exceptional Conditions

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary5.3MEDIUMCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
3.1CNACVSS5.3MEDIUMCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

CVSS v3.1 Breakdown

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

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Mattermost Mattermost Server All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Mattermost Mattermost affected 11.9.0 11.9.0 semver Not specified
CNA Mattermost Mattermost affected 11.8.0 11.8.4 semver Not specified
CNA Mattermost Mattermost affected 11.7.0 11.7.7 semver Not specified
CNA Mattermost Mattermost affected 10.11.0 10.11.22 semver Not specified
CNA Mattermost Mattermost unaffected 11.10.0 Not specified
CNA Mattermost Mattermost unaffected 11.9.1 Not specified
CNA Mattermost Mattermost unaffected 11.8.5 Not specified
CNA Mattermost Mattermost unaffected 11.7.8 Not specified
CNA Mattermost Mattermost unaffected 10.11.23 Not specified

References

ReferenceSourceLinkTags
mattermost.com/security-updates [email protected] mattermost.com Vendor Advisory
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: kamil-sawicki (en)

Additional Advisory Data

Solutions

CNA: Update Mattermost to versions 11.10.0, 11.9.1, 11.8.5, 11.7.8, 10.11.23 or higher.

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