---
id: CVE-2026-18799
title: >-
  Mattermost Server fails to restrict Slack import account binding, allowing
  team admins to forge posts as existing users
summary: >-
  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 p…
severity: medium
cvss: 4.3
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N'
cvssSource: cna
cwe:
  - CWE-862
vendor: Mattermost
product: Mattermost
affected:
  - Mattermost >= 11.10.0 <= 11.10.0
  - Mattermost >= 11.7.0 <= 11.7.9
  - Mattermost >= 11.9.0 <= 11.9.1
  - Mattermost >= 11.8.0 <= 11.8.5
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-10-16T11:28:31.537040Z'
published: '2026-10-16'
updated: '2026-10-16'
sourceUpdated: '2026-10-16T11:28:45.200Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-18799'
references:
  - url: 'https://mattermost.com/security-updates'
    label: MMSA-2026-00757
tags:
  - cve.org
ingestedAt: '2026-10-16T08:02:23.120Z'
---

## Overview

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

- `Mattermost >= 11.10.0 <= 11.10.0`
- `Mattermost >= 11.7.0 <= 11.7.9`
- `Mattermost >= 11.9.0 <= 11.9.1`
- `Mattermost >= 11.8.0 <= 11.8.5`

## Remediation

Update Mattermost to versions 11.11.0, 11.10.1, 11.7.10, 11.9.2, 11.8.6 or higher.