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

Quick assessment

Affected
Mattermost Mattermost
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Mattermost 版本 11.9.x（≤ 11.9.0）、11.8.x（≤ 11.8.4）、11.7.x（≤ 11.7.7）以及 10.11.x（≤ 10.11.22）未能验证 Microsoft Graph Webhook 通知负载中的空（null）条目，这使得未经身份验证的攻击者可以通过向公共 Webhook 端点发送一个经过构造的 {{POST}} 请求，导致 Microsoft 日历插件进程崩溃，从而使该实例上的所有用户无法使用日历集成服务。 Mattermost 安全公告 ID：MMSA-2026-0

CVSS 5.3 · Medium

Possible ATT&CK Techniques 1 AI

T1032
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I. Basic Information for CVE-2026-10556

Vulnerability Information
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Vulnerability Title
Unauthenticated webhook request with null notification entry could crash the Microsoft Calendar plugin.
Source: CVE Program / CVE List V5
Vulnerability 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
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
Source: CVE Program / CVE List V5
Vulnerability Type
对因果或异常条件的不恰当检查
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
Mattermost Mattermost 11.9.0 ~ 11.9.0 -

II. Public POCs for CVE-2026-10556

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III. Intelligence Information for CVE-2026-10556

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Same Patch Batch · Mattermost · 2026-09-14 · 11 CVEs total

CVE-2026-8821 7.1 HIGH Playbooks run owner channel membership permission bypass
CVE-2026-9812 6.5 MEDIUM Missing property field ownership validation in Playbooks run property update endpoint
CVE-2026-5132 6.5 MEDIUM Unbounded zlib decompression in Calls SDP WebSocket messages
CVE-2026-15814 6.5 MEDIUM Uploading a crafted image causes excessive memory allocation in the Mattermost Server
CVE-2026-10542 5.0 MEDIUM Playbooks channel action update validation issue
CVE-2026-13417 4.3 MEDIUM Boards plugin denial of service via unvalidated block fields.properties
CVE-2026-14344 4.3 MEDIUM Inconsistent authorization checks in Mattermost Boards endpoints
CVE-2026-12882 4.3 MEDIUM Mattermost Markdown autolink parsing denial of service
CVE-2026-11993 4.3 MEDIUM Fix authenticated members disabling file content indexing server-wide via extraction pool
CVE-2026-14259 4.3 MEDIUM Board archive import bypasses team board creation permissions

IV. Related Vulnerabilities

Same product: Mattermost
Same vendor: Mattermost
Same weakness: CWE-754

V. Comments for CVE-2026-10556

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