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TI-2026-08603 Aliases CVE-2026-9816GHSA-h2c9-x337-f543

Insufficient server-side validation of board member role fields permits privilege escalation

Priority LOW 36CWE-863Published · Updated

Key facts

Attack vector
Network
Privileges required
Low
User interaction
None
Attack complexity
Low
Impact type
Security bypass
Confidentiality · integrity · availability
Low · High · High
Fix status
Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA. Impact type from CWE-863.

Summary

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

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.0035 28th percentileScored 2026-11-07
Fix Fixed Mattermost (Mattermost): 11.9.0, 11.7.7, 10.11.22 +1

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
None
Automatable
No
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
MattermostMattermost11.7.0 – 11.7.6; 10.11.0 – 10.11.21; 11.8.0 – 11.8.311.9.0, 11.7.7, 10.11.22, 11.8.4Patch availableCNA
  • Mattermost VendorMattermost Affected 11.7.0 – 11.7.6; 10.11.0 – 10.11.21; 11.8.0 – 11.8.3 Fixed in 11.9.0, 11.7.7, 10.11.22, 11.8.4 Patch available CNA

CPE configurations NVD · 1

CPEVersionsRoleVendor
cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*10.11.0 to before 10.11.22; 11.7.0 to before 11.7.7; 11.8.0 to before 11.8.4VulnerableMattermost
  • cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:* Versions 10.11.0 to before 10.11.22; 11.7.0 to before 11.7.7; 11.8.0 to before 11.8.4 Vulnerable ·Mattermost

Remediation

  • Fix · CNA Mattermost (Mattermost) Fixed in 11.9.0, 11.7.7, 10.11.22, 11.8.4

Timeline

  1. 2026-10-02 Threatist record created from CVE-2026-9816
  2. 2026-10-03 Alias added: GHSA-h2c9-x337-f543

References

Advisory 1

  • TI-2025-18933 Arbitrary file read in Mattermost Boards via import & export board archive CVSS 3.1CRITICAL 9.9
  • TI-2025-04465 Path Traversal Leading to RCE by Any Authenticated Mattermost User CVSS 3.1CRITICAL 9.9
  • TI-2017-14068 An issue was discovered in Mattermost Server before 3.6.2. CVSS 3.1CRITICAL 9.8
  • TI-2017-14060 An issue was discovered in Mattermost Server before 3.8.2, 3.7.5, and 3.6.7. CVSS 3.1CRITICAL 9.8
  • TI-2017-14048 An issue was discovered in Mattermost Server before 4.1.0, 4.0.4, and 3.10.3. CVSS 3.1CRITICAL 9.8
  • TI-2025-14360 Arbitrary file read via block duplication in Mattermost Boards CVSS 3.1CRITICAL 9.9
See all 505 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences