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

> An unauthenticated attacker can crash the Microsoft Calendar plugin by sending a crafted webhook request with null entries, denying calendar services.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 14 Sep 2026
- **Updated:** 14 Sep 2026
- **Also known as:** GHSA-56v5-m3m2-x7qh, MINI-78pp-q87v-w78r, MINI-gpxw-8c6h-v6c6

## What is CVE-2026-10556?

CVE-2026-10556 is a medium-severity vulnerability in Mattermost Mattermost Server, published on 14 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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

Weakness: CWE-476 (NULL Pointer Dereference), CWE-754 (Improper Check for Unusual or Exceptional Conditions)

## Which versions are affected by CVE-2026-10556?

It affects 1 product.


Products: Mattermost Mattermost Server.

## How do I fix CVE-2026-10556?

Upgrade to a fixed version.

## Is CVE-2026-10556 being exploited?

There is no known exploitation of CVE-2026-10556 in the wild and no public exploit code.

## Other vulnerabilities in Mattermost Mattermost Server

- [CVE-2025-12419](https://insights.sbomify.com/CVE-2025-12419/): Account takeover on OAuth/OpenID-enabled servers (critical 9.9)
- [CVE-2025-12421](https://insights.sbomify.com/CVE-2025-12421/): Account Takeover via Code Exchange Endpoint (critical 9.9)
- [CVE-2025-4981](https://insights.sbomify.com/CVE-2025-4981/): Path Traversal Leading to RCE by Any Authenticated Mattermost User (critical 9.9)
- [CVE-2025-20051](https://insights.sbomify.com/CVE-2025-20051/): Arbitrary file read via block duplication in Mattermost Boards (critical 9.9)
- [CVE-2025-25279](https://insights.sbomify.com/CVE-2025-25279/): Arbitrary file read in Mattermost Boards via import & export board archive (critical 9.9)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-10556 and fix it if it is affected.

What it is: CVE-2026-10556 is a medium-severity vulnerability in Mattermost Mattermost Server, published on 14 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-10556 in the wild and no public exploit code.

Affected product: Mattermost Mattermost Server. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-10556/ (Markdown: https://insights.sbomify.com/CVE-2026-10556/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-10556
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-10556
- OSV: https://osv.dev/vulnerability/CVE-2026-10556

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-10556/
Summaries are generated and may contain mistakes; the official description above is the record.