● Medium · CVSS 4.6

How to Fix CVE-2026-1628: Inclusion of functionality from untrusted control sphere in Mattermost

Q: What is the CVSS score for CVE-2026-1628?

4.6 (Medium). Vector: `CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:L/A:N`.

Q: Where is the official advisory for CVE-2026-1628?

The vendor advisory is at [https://mattermost.com/security-updates](https://mattermost.com/security-updates). The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-1628.

By Sai Kiran Pandrala. Last verified: 2026-05-25.

CVE-2026-1628 is a inclusion of functionality from untrusted control sphere in Mattermost Mattermost. The fix is to upgrade to Mattermost 5.13.4.0.

⚡ At a glance


Severity	4.6 (Medium)
Actively exploited?	No public listing in CISA KEV
Affected	Mattermost 0 to <=5.13.3
Fixed in	Mattermost 5.13.4.0
Type (CWE)	CWE-829: Inclusion of Functionality from Untrusted Control Sphere

What is CVE-2026-1628?

Mattermost Desktop App versions <=5.13.3 fail to attach listeners restricting navigation to external sites within the Mattermost app which allows a malicious server to expose preload script functionality to untrusted servers via having a user open an external link in their Mattermost server. Mattermost Advisory ID: MMSA-2026-00596 The CVSS base score is 4.6 (Medium). The official advisory is at https://mattermost.com/security-updates.

Am I affected?

Check the version of Mattermost you are running and compare it against the Affected row above (Mattermost 0 to <=5.13.3). If your build sits inside the affected range, you must patch.

Run the version check that fits your platform:


# Linux package check
dpkg -s mattermost 2>/dev/null | grep -i ^Version
rpm -q mattermost 2>/dev/null
command -v mattermost >/dev/null && mattermost --version 2>/dev/null


# Windows (PowerShell)
Get-Package -Name "*Mattermost*" -ErrorAction SilentlyContinue | Select-Object Name, Version
winget list --name "Mattermost" 2>$null

How to fix CVE-2026-1628

Upgrade Mattermost to a patched build: Mattermost 5.13.4.0. The vendor advisory is the source of truth for the exact fixed version.

Ubuntu / Debian


sudo apt-get update
sudo apt-get install --only-upgrade mattermost
dpkg -s mattermost | grep -i ^Version

RHEL / CentOS / Rocky / AlmaLinux


sudo dnf upgrade --refresh mattermost -y
# or for older releases:
sudo yum update mattermost -y
rpm -q mattermost

SUSE / openSUSE


sudo zypper refresh
sudo zypper update mattermost
rpm -q mattermost

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-1628 affecting Mattermost
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-1628-fix-$(Get-Date -Format yyyyMMdd-HHmmss).log"
New-Item -ItemType Directory -Force (Split-Path $LogPath) | Out-Null
Start-Transcript -Path $LogPath -Append

try {
    Write-Host "[1/4] Detect installed version"
    $pkg = Get-Package -Name "*Mattermost*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

    Write-Host "[2/4] Backup configuration"
    $backup = "C:\Backup\mattermost-$(Get-Date -Format yyyyMMdd)"
    New-Item -ItemType Directory -Force $backup | Out-Null
    Get-ChildItem "C:\ProgramData\Mattermost" -ErrorAction SilentlyContinue |
        Copy-Item -Destination $backup -Recurse -Force -ErrorAction SilentlyContinue

    Write-Host "[3/4] Apply the upgrade to Mattermost 5.13.4.0"
    winget upgrade --name "Mattermost" --silent --accept-source-agreements --accept-package-agreements
    if ($LASTEXITCODE -ne 0) {
        # Fallback: pull latest via OS update channel
        Install-Module -Name PSWindowsUpdate -Force -SkipPublisherCheck -ErrorAction SilentlyContinue
        Import-Module PSWindowsUpdate
        Install-WindowsUpdate -MicrosoftUpdate -AcceptAll -IgnoreReboot
    }

    Write-Host "[4/4] Verify the patched build"
    winget list --name "Mattermost"
    Write-Host "Patch applied. Reboot if prompted."
    exit 0
} catch {
    Write-Error "Patch failed: $_"
    exit 1
} finally {
    Stop-Transcript
}

Complete Bash remediation script (Linux)


#!/usr/bin/env bash
# Fix script for CVE-2026-1628 affecting Mattermost
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-1628-fix-$(date +%Y%m%d-%H%M%S).log"
exec > >(tee -a "$LOG") 2>&1

echo "[1/4] Detect installed version"
if command -v dpkg >/dev/null; then
    dpkg -s mattermost 2>/dev/null | grep -i ^Version || echo "mattermost not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q mattermost || echo "mattermost not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-1628-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/mattermost /etc/mattermost.d /etc/mattermost.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Apply the upgrade (target: Mattermost 5.13.4.0)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y mattermost
elif command -v dnf >/dev/null; then
    dnf upgrade --refresh -y mattermost
elif command -v yum >/dev/null; then
    yum update -y mattermost
elif command -v zypper >/dev/null; then
    zypper --non-interactive update mattermost
fi

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s mattermost 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q mattermost
fi
echo "Done. Restart any service that loaded the old library."

If you can't patch immediately

Apply at least one of the following inline controls until you can deploy the patched build. None replace the upgrade.

Restrict exposure with nftables (Linux)


# Allow only trusted CIDR to reach the affected service ports
sudo nft add table inet filter 2>/dev/null || true
sudo nft 'add chain inet filter input { type filter hook input priority 0 ; }' 2>/dev/null || true
sudo nft 'add rule inet filter input tcp dport {80, 443} ip saddr != 10.0.0.0/8 drop'
sudo nft list ruleset

Block at the host firewall (Windows)


New-NetFirewallRule -DisplayName "Block-CVE-2026-1628" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-1628"

Disable the affected service (Linux)


sudo systemctl stop mattermost 2>/dev/null || true
sudo systemctl disable mattermost 2>/dev/null || true

If the vendor advisory lists an official workaround, prefer that wording verbatim. If no workaround is published, the only safe remediation is the patch.

How to verify the fix worked

After upgrading, confirm the installed version matches the patched build and that no old library is still loaded by a long-running process.


# Linux
dpkg -s mattermost 2>/dev/null | grep -i ^Version
rpm -q mattermost 2>/dev/null || true
# Pid map check for old library handles
sudo lsof +c0 2>/dev/null | grep -i "DEL.*lib" || true


# Windows
Get-Package -Name "*Mattermost*" | Select-Object Name, Version
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Expected: the reported version is at or above Mattermost 5.13.4.0. Restart the affected service (systemctl restart <service> on Linux, or restart the Windows service) so it loads the patched binary.

Frequently asked questions

Other vulnerabilities in the same area that are worth patching alongside this one:

How to Fix CVE-2026-7030: Buffer Overflow in F456 — Buffer Overflow in F456
How to Fix CVE-2026-34926: Relative Path Traversal in TrendAI Apex One — Relative Path Traversal in TrendAI Apex One
How to Fix CVE-2026-33633: Path Traversal in kitty , Path Traversal in kitty
How to Fix CVE-2026-8415: Cross-Site Request Forgery in Concrete CMS , Cross-Site Request Forgery in Concrete CMS
How to Fix CVE-2026-45229: Critical Vulnerability in quark-auto-save , Critical Vulnerability in quark-auto-save

Is CVE-2026-1628 actively exploited?

There is no public confirmation of exploitation in the wild listed in CISA KEV at the time of this writing. Patch anyway. Public exploits commonly follow disclosure within weeks.

Do I need to reboot after patching CVE-2026-1628?

For kernel and OS-level updates, yes. For most userland packages a systemctl restart <service> is enough on Linux, and restarting the Windows service or app on Windows. Any process that loaded the old library keeps using it until restarted.

What is the CVSS score for CVE-2026-1628?

4.6 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:L/A:N.

Where is the official advisory for CVE-2026-1628?

The vendor advisory is at https://mattermost.com/security-updates. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-1628.

References

Official vendor advisory: https://mattermost.com/security-updates
NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-1628
CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog

*Written by Sai Kiran Pandrala. Assembled from the official vendor advisory, NVD record, and CISA KEV listing on 2026-05-25. Always confirm against the vendor's advisory before applying changes in production.*