Authorization Flaw in Mattermost (CVE-2026-9816)
Summary
Vulnerability in Mattermost (CVE-2026-9816). Data can be tampered with by attackers. Exploitable via POST /api/v2/boards/{boardID}/members.
Original description
The description as published by the source.
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: NIST National Vulnerability Database NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-9816 has been found in Mattermost.
Attackers can target a specific entry point like POST /api/v2/boards/{boardID}/members over the network to misuse the product.
Data can be tampered with by attackers. CVSS score: 8.3/10.
What to do: apply the vendor's official patched release.
If unsure, ask your IT team or search "Mattermost CVE-2026-9816" on the vendor's site.
CVE-2026-9816 (Mattermost) — CWE-863 / CVSS v3 8.3
Attack vector: remote (network-reachable) / no user interaction
Attack surface: POST /api/v2/boards/{boardID}/members / POST /api/v2/teams/{teamID}/archive/import..
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
Incorrect authorization (CWE-863) exists in Mattermost. Attackers reach the vulnerable code path via POST /api/v2/boards/{boardID}/members.
Check the vendor advisory for the affected versions of Mattermost. Attack surface: POST /api/v2/boards/{boardID}/members / POST /api/v2/teams/{teamID}/archive/import...
Severity: High (CVSS 8.3/10). Data can be tampered with by attackers.
Update to the patched release per vendor advisory.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual POST /api/v2/boards/{boardID}/members requests with malformed payloads or SQL meta-characters. Run grep -r 'Mattermost' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
grep -r 'Mattermost' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
Mattermostを grep し、稼働しているサービス・バージョンを把握する。 -
2Hunt for indicators of compromise detect
grep 'POST /api/v2/boards/{boardID}/members' /var/log/nginx/access.log | grep -E '(unusual_payload|sqli_pattern)'アクセスログで
POST /api/v2/boards/{boardID}/membersへの異常なリクエスト (不正な認証ヘッダ・SQLメタ文字)を過去 30〜90日分捜索。WAF/SIEM があれば該当パスのアラート発火履歴を確認。 -
3Post-deployment verification verify
Replay attack against POST /api/v2/boards/{boardID}/members on staging to confirm patch closes the vectorパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番では Step 2 と同じログクエリでアラート再発が無いか継続監視。
References
- patch responsibledisclosure@mattermost.com
- web nvd.nist.gov
- web github.com