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CVE-2026-8821— Playbooks run owner channel membership permission bypass

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 在创建 playbook（运行剧本）时未对频道成员管理权限进行有效校验，导致已认证的频道成员可以通过“运行所有者”（run owner）字段将任意用户添加到受限频道中。Mattermost 安全公告编号：MMSA-2026-00677

CVSS 7.1 · High

Possible ATT&CK Techniques 1 AI

T1078 · Valid Accounts
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I. Basic Information for CVE-2026-8821

Vulnerability Information
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Vulnerability Title
Playbooks run owner channel membership permission bypass
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 channel member-management permission during playbook run creation, allowing an authenticated channel member to add an arbitrary user to a restricted channel via the run owner field.. Mattermost Advisory ID: MMSA-2026-00677
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
授权机制缺失
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-2026-8821

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III. Intelligence Information for CVE-2026-8821

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

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-2026-10556 5.3 MEDIUM Unauthenticated webhook request with null notification entry could crash the Microsoft Cal
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-862

V. Comments for CVE-2026-8821

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