---
id: "CVE-2026-18799"
kind: "vulnerability"
aliases: ["CVE-2026-18799"]
title: "Mattermost: Slack import lets team admins impersonate existing users"
severity: "medium"
cvss: 4.3
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["chat", "mattermost", "package", "self-hosted"]
not_affected_if: ["default-config", "versions-outside-range"]
published_at: "2026-10-16T08:17:13+00:00"
first_seen_at: "2026-10-23T21:57:12+00:00"
updated_at: "2026-10-25T04:41:09+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-18799", "first_seen_at": "2026-10-23T21:57:12+00:00"}]
---

# Mattermost: Slack import lets team admins impersonate existing users

Mattermost's Slack import does not limit account binding to imports started by a system administrator. A team admin can craft a Slack export containing other users' email addresses to force-add those users to their team and post messages attributed to them.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the running Mattermost server version with `mattermost version` or the System Console; affected builds are 11.7.x up to `11.7.9`, 11.8.x up to `11.8.5`, 11.9.x up to `11.9.1`, and 11.10.x up to `11.10.0`.
- Check whether team admins can run Slack imports in the System Console under Data Management.

## Fix

- Upgrade Mattermost to a fixed release listed in advisory MMSA-2026-00757 (the fixed versions are not stated in the sources; check the advisory).
- Until upgraded, restrict team admin use of Slack import.

## Timeline

- 2026-10-23T21:57:12+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-18799

## From the sources

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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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_