# CVE-2026-10556: Unauthenticated webhook request with null notification entry could crash the Microsoft Calendar plugin.

> CVE-2026-10556: Unauthenticated webhook request with null notification entry could crash the Microsoft Calendar plugin.. CVSS 5.3 (CVSS 3.1).

Canonical: https://rewindex.com/cve/CVE-2026-10556
Last updated: 2026-10-07T16:25:31Z

- Published: 2026-09-14 10:46 UTC · Updated: 2026-10-07 16:25 UTC · Assigner: Mattermost
- CVSS: 5.3 (CVSS 3.1) · Severity: Medium · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- EPSS: 0.42%, 36th percentile (scored 2026-10-25)
- CISA KEV: no
- CWE: CWE-754

## Description

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

## Affected vendors and products

- Mattermost: Mattermost

## Tracked companies

- [Mattermost](https://rewindex.com/company/mattermost.md)

## References

- [CVE record on cve.org](https://www.cve.org/CVERecord?id=CVE-2026-10556)
- [NVD entry](https://nvd.nist.gov/vuln/detail/CVE-2026-10556)
- [https://mattermost.com/security-updates](https://mattermost.com/security-updates)

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_Sources: CVE records from the CVE Program (cve.org). CVE® is a registered trademark of The MITRE Corporation. (https://www.cve.org/) This product uses data from the NVD API but is not endorsed or certified by the NVD. (https://nvd.nist.gov/) Known Exploited Vulnerabilities catalog from CISA. (https://www.cisa.gov/known-exploited-vulnerabilities-catalog) EPSS scores from FIRST (Exploit Prediction Scoring System). (https://www.first.org/epss/)_


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Canonical: https://rewindex.com/cve/CVE-2026-10556 · Markdown: https://rewindex.com/cve/CVE-2026-10556.md · JSON: https://rewindex.com/api/v1/cves/CVE-2026-10556 · Data licensed CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) · Generated 2026-10-26T04:06:10.110Z