⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● Critical · CVSS 10

How to Fix CVE-2026-4370: Improper certificate validation in Juju

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

*By Sai Kiran Pandrala*

Last verified: 2026-05-25

⚡ At a glance
SeverityCVSS 10, Critical
Actively exploited?No
AffectedCanonical Juju (3.2.0 < 3.6.20, 4.0 < 4.0.4)
Fixed in3.6.20, 4.0.4
Type (CWE)CWE-295: Improper certificate validation

CVE-2026-4370 is a improper certificate validation in Canonical Juju. The fix is to upgrade to 3.6.20, 4.0.4 and apply the runnable commands below.

What is CVE-2026-4370?

A vulnerability was identified in Juju from version 3.2.0 until 3.6.19 and from version 4.0 until 4.0.4, where the internal Dqlite database cluster fails to perform proper TLS client and server authentication. Specifically, the Juju controller's database endpoint does not validate client certificates when a new node attempts to join the cluster. An unauthenticated attacker with network reachability to the Juju controller's Dqlite port can exploit this flaw to join the database cluster.

In practical terms, a successful attacker gets compromise of the affected component as described in the vendor advisory. There is no confirmed in-the-wild exploitation listed in CISA's KEV catalog at the time of writing, but the CVSS rating still warrants prompt patching.

Am I affected?

You are affected if you run Canonical Juju at a version listed in the Affected row above. Probe your installed build with the commands below.


# Confirm the installed version via your package manager
dpkg -l | grep -i juju   # Debian/Ubuntu
rpm -qa | grep -i juju   # RHEL/CentOS/Rocky

How to fix CVE-2026-4370

The primary fix is to upgrade to the patched build listed in the Fixed in row above (3.6.20, 4.0.4). Pick the platform that matches your install and run the commands below.

Linux (Ubuntu / Debian)


sudo apt-get update
sudo apt-get install --only-upgrade juju
# Confirm the installed version meets or exceeds 3.6.20
dpkg -s juju | grep ^Version

Linux (RHEL / CentOS / Rocky)


sudo dnf upgrade --security juju -y
rpm -q juju

Windows (PowerShell, admin)


winget upgrade --id 'Canonical.Juju' --silent --accept-source-agreements --accept-package-agreements
# If winget doesn't know the product, download the patched installer from the vendor and:
Start-Process -FilePath "$env:TEMP\Juju-3.6.20.msi" -ArgumentList '/qn /norestart' -Wait

PowerShell script (Windows): detect, back up, upgrade, verify, log


# Run as Administrator
$ErrorActionPreference = 'Stop'
$log = "$env:ProgramData\Juju-Patch-CVE-2026-4370.log"
function Write-Log($msg) { "$(Get-Date -Format s)  $msg" | Tee-Object -FilePath $log -Append }

Write-Log "Starting CVE-2026-4370 remediation for Canonical Juju"

# 1. Detect: replace the path/version probe with one valid for your install
$installed = (Get-WmiObject -Class Win32_Product |
    Where-Object { $_.Name -like '*Juju*' } |
    Select-Object -First 1 -ExpandProperty Version)
Write-Log "Detected version: $installed"

if (-not $installed) {
    Write-Log "Product not installed on this host; nothing to do."
    return
}
if ([version]$installed -ge [version]'3.6.20') {
    Write-Log "Already at fixed version $installed; no action needed."
    return
}

# 2. Backup configuration to a timestamped folder
$backup = "$env:ProgramData\Juju-Backup-$(Get-Date -Format yyyyMMdd-HHmm)"
New-Item -ItemType Directory -Path $backup -Force | Out-Null
$src = "$env:ProgramFiles\Canonical\Juju"
if (Test-Path $src) { Copy-Item -Path $src -Destination $backup -Recurse -Force }
Write-Log "Backed up config to $backup"

# 3. Apply the patched installer
$installer = "$env:TEMP\Juju-3.6.20.msi"
if (-not (Test-Path $installer)) {
    throw "Patched installer not found at $installer. Stage it from your software repo first."
}
Start-Process msiexec.exe -ArgumentList "/i `"$installer`" /qn /norestart" -Wait
Write-Log "Installer finished"

# 4. Verify
$verify = (Get-WmiObject -Class Win32_Product |
    Where-Object { $_.Name -like '*Juju*' } |
    Select-Object -First 1 -ExpandProperty Version)
if ([version]$verify -ge [version]'3.6.20') {
    Write-Log "SUCCESS: now at $verify (>= 3.6.20)"
} else {
    Write-Log "FAILURE: still at $verify after install"
    exit 1
}

Bash script (Linux): detect, back up, upgrade, verify, log


#!/usr/bin/env bash
set -euo pipefail
LOG=/var/log/juju-patch-cve-2026-4370.log
log()  { echo "$(date -Iseconds)  $*" | tee -a "$LOG"; }

log "Starting CVE-2026-4370 remediation for Canonical Juju"

# 1. Detect installed version (works for deb and rpm packages)
if command -v dpkg >/dev/null && dpkg -s juju >/dev/null 2>&1; then
    CURRENT=$(dpkg-query -W -f='${Version}' juju)
    PKG_MGR=apt
elif command -v rpm >/dev/null && rpm -q juju >/dev/null 2>&1; then
    CURRENT=$(rpm -q --queryformat '%{VERSION}' juju)
    PKG_MGR=dnf
else
    log "juju not installed via apt or rpm; check your package manager or vendor instructions."
    exit 0
fi
log "Detected: juju=$CURRENT (manager=$PKG_MGR)"

# 2. Backup config
BACKUP=/var/backups/juju-$(date +%Y%m%d-%H%M)
mkdir -p "$BACKUP"
for d in /etc/juju /etc/${pkg%%-*} ; do
    [ -d "$d" ] && cp -a "$d" "$BACKUP/" && log "Backed up $d to $BACKUP"
done

# 3. Upgrade
if [ "$PKG_MGR" = apt ]; then
    sudo apt-get update -y
    sudo apt-get install --only-upgrade -y juju
else
    sudo dnf upgrade --security -y juju
fi

# 4. Verify
if [ "$PKG_MGR" = apt ]; then
    NEW=$(dpkg-query -W -f='${Version}' juju)
else
    NEW=$(rpm -q --queryformat '%{VERSION}' juju)
fi
log "After upgrade: $NEW"
log "Done. Compare $NEW against 3.6.20 and restart the affected service if needed."

If you cannot patch immediately

These are runnable hardening commands. They reduce blast radius but they are not a replacement for the vendor patch.

No official vendor workaround is published for this CVE; patching is the documented fix. The runnable hardening below is generic defense in depth, not a substitute for the patch.

Restrict the affected service to trusted networks (Linux):


# Vendor advisory: https://github.com/juju/juju/security/advisories/GHSA-gvrj-cjch-728p
sudo iptables -A INPUT -p tcp --dport <port> -s 10.0.0.0/24 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport <port> -j DROP

Windows Firewall equivalent:


# Vendor advisory: https://github.com/juju/juju/security/advisories/GHSA-gvrj-cjch-728p
New-NetFirewallRule -DisplayName 'Allow affected service from admin subnet' -Direction Inbound -Action Allow -Protocol TCP -LocalPort <port> -RemoteAddress 10.0.0.0/24
New-NetFirewallRule -DisplayName 'Block affected service from everywhere else' -Direction Inbound -Action Block -Protocol TCP -LocalPort <port>

How to verify the fix worked

Run the version probe again and confirm the running build matches the Fixed in row above.


dpkg -l | grep -i "juju"   # Debian/Ubuntu
rpm -qa | grep -i "juju"   # RHEL/CentOS/Rocky

Expected output: the package version should meet or exceed 3.6.20.

Re-run any vulnerability scanner you used previously and confirm the finding for CVE-2026-4370 has cleared. Sweep your logs for indicators of compromise listed in the vendor or CISA advisory, especially if the system was internet-reachable during the disclosure window.

Frequently asked questions

Is CVE-2026-4370 being actively exploited?

Not at the time of writing. It is not listed in CISA's Known Exploited Vulnerabilities catalog. That status can change, so monitor the vendor advisory and the KEV catalog if the system is exposed.

How severe is CVE-2026-4370?

CVSS rates it 10 (Critical). Use that score to set your patch priority alongside the other items in your queue.

Do I have to take Juju offline to apply the patch?

It depends on the deployment. High-availability or clustered installs can usually patch one node at a time with no full outage. Standalone installs typically need a short restart. Always follow the vendor's documented upgrade steps.

What if my vulnerability scanner still flags CVE-2026-4370 after I patch?

Re-run the scan after a service restart, then confirm the scanner's plugin set is up to date. Some scanners detect by banner version only and lag the official fix metadata by a release.

References

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