CVE-2026-10556Network attack vectorHinoki check available

Service interruption via null webhook notification entry

Published Sep 14, 2026 · Updated Sep 14, 2026

Denial of service in affected Mattermost releases allows remote attackers to crash the Microsoft Calendar plugin via a crafted webhook. The Microsoft Graph webhook handler iterates over the notification array and dereferences each entry without rejecting null values. The public endpoint requires no authentication, and one crafted POST interrupts calendar integration for every user on the instance.

CVSS severityUnavailable
Unscored
EPSS probability0.25%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkAvailable
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Denial of service in affected Mattermost releases allows remote attackers to crash the Microsoft Calendar plugin via a crafted webhook. The Microsoft Graph webhook handler iterates over the notification array and dereferences each entry without rejecting null values. The public endpoint requires no authentication, and one crafted POST interrupts calendar integration for every user on the instance.

The record

CVE
CVE-2026-10556
Published
Sep 14, 2026
Updated
Sep 14, 2026
Vendor
Mattermost
Product
Mattermost
Classifications
T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 14, 2026CVE publishedPublication date reported by the CVE source.
  2. Sep 14, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=10.11.0 <=10.11.22
  2. Affected versionversion=11.7.0 <=11.7.7
  3. Affected versionversion=11.8.0 <=11.8.4
  4. Affected versionversion=11.9.0 <=11.9.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mattermost · Mattermostversion=10.11.0 <=10.11.22; version=11.7.0 <=11.7.7; version=11.8.0 <=11.8.4; version=11.9.0 <=11.9.0

Published CVSS scores

No CVSS assessment is available in this record.

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo