CVE-2026-18799 — Mattermost Server fails to restrict Slack import account binding, allowing team admins to forge posts as existing users

Affected: Mattermost

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

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CVE-2026-18799 — Mattermost Server fails to restrict Slack import account binding, allowing team admins to forge posts as existing users