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CVE-2026-9816— Insufficient server-side validation of board member role fields permits privilege escalation

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是美国Mattermost公司开源的一个开源协作平台。 Mattermost 11.7.6及之前的11.7.x版本、10.11.21及之前的10.11.x版本和11.8.3及之前的11.8.x版本存在授权问题漏洞，该漏洞源于服务端在插入和归档导入路径中未能验证BoardMember.Scheme*字段，可能导致看板编辑者或非来宾团队成员通过POST /api/v2/boards/{boardID}/members和POST /api/v2/teams/{teamID}/archive/

CVSS 8.3 · High EPSS 0.24% · P16

Possible ATT&CK Techniques 1 AI

T1078 · Valid Accounts

Affected Version Matrix 7

VendorProduct Version RangeStatus
Mattermost Mattermost 11.7.0≤ 11.7.6 affected
10.11.0≤ 10.11.21 affected
11.8.0≤ 11.8.3 affected
11.9.0 unaffected
11.7.7 unaffected
10.11.22 unaffected
11.8.4 unaffected
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I. Basic Information for CVE-2026-9816

Vulnerability Information
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Vulnerability Title
Insufficient server-side validation of board member role fields permits privilege escalation
Source: CVE Program / CVE List V5
Vulnerability Description
Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to validate BoardMember.Scheme* fields server-side on insert and archive-import paths which allows a board editor or non-guest team member to grant board admin to arbitrary users via POST /api/v2/boards/{boardID}/members and POST /api/v2/teams/{teamID}/archive/import.. Mattermost Advisory ID: MMSA-2026-00685
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
授权机制不正确
Source: CVE Program / CVE List V5
Vulnerability Title
Mattermost 授权问题漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Mattermost是美国Mattermost公司开源的一个开源协作平台。 Mattermost 11.7.6及之前的11.7.x版本、10.11.21及之前的10.11.x版本和11.8.3及之前的11.8.x版本存在授权问题漏洞，该漏洞源于服务端在插入和归档导入路径中未能验证BoardMember.Scheme*字段，可能导致看板编辑者或非来宾团队成员通过POST /api/v2/boards/{boardID}/members和POST /api/v2/teams/{teamID}/archive/
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
Mattermost Mattermost 11.7.0 ~ 11.7.6 -

II. Public POCs for CVE-2026-9816

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III. Intelligence Information for CVE-2026-9816

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Same Patch Batch · Mattermost · 2026-08-17 · 13 CVEs total

CVE-2026-9859 6.5 MEDIUM Mattermost Boards plugin didn’t enforce role-based authorization on board channel link all
CVE-2026-10080 6.5 MEDIUM Boards plugin panics on WebSocket command with non-string field types
CVE-2026-10527 6.3 MEDIUM Boards plugin retains Board Admin rights for users demoted to System Guest
CVE-2026-16048 6.3 MEDIUM Channel member roles accept out-of-scope roles
CVE-2026-16047 4.3 MEDIUM Board channel linking without read channel permission validation
CVE-2026-15754 4.2 MEDIUM Missing per-channel team-scope check in ABAC access control policy unassign allows cross-t
CVE-2026-16044 3.9 LOW Insufficient validation of guest board admin privileges on archive import
CVE-2026-16049 3.9 LOW _GitLab Plugin allows cross-channel post injection and phishing via missing channel permis
CVE-2026-75587 3.6 LOW Plaintext pre-auth secret exposure via Desktop App diagnostics report
CVE-2026-9693 3.5 LOW Mattermost thread memberships persist after team removal, exposing private channel thread
CVE-2026-16046 3.5 LOW Missing run-state validation on finished playbook runs
CVE-2026-16045 2.7 LOW Delegated OAuth tokens could revoke unrelated OAuth application authorizations

IV. Related Vulnerabilities

Same product: Mattermost
Same vendor: Mattermost
Same weakness: CWE-863

V. Comments for CVE-2026-9816

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