Medium · 4.3 CVE-2026-18799 EUVD-2026-102751GHSA-f4q7-3g3r-qrg8

Mattermost Server fails to restrict Slack import account binding, allowing team admins to forge posts as existing users

Published Oct 16, 2026 · Updated Oct 16, 2026 · Assigned by Mattermost

Description

Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to restrict Slack import account binding to system administrator-initiated imports which allows an authenticated attacker with team admin privileges to force-add existing Mattermost users to their team and forge posts attributed to those users' identities via a crafted Slack export containing target users' email addresses.. Mattermost Advisory ID: MMSA-2026-00757

Affected products

Vendor	Product	Affected	Fixed
Mattermost	Mattermost	11.10.0 – ≤ 11.10.0, 11.7.0 – ≤ 11.7.9, 11.9.0 – ≤ 11.9.1, 11.8.0 – ≤ 11.8.5	11.11.0

Advisories 1

WID-SEC-2026-4009 Mattermost: Mehrere Schwachstellen BSI CERT-Bund (WID) · Oct 15, 2026

References

MMSA-2026-00757 vendor-advisory