⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● Medium · CVSS 4.4

How to Fix CVE-2026-34726: Copier _subdirectory allows template root escape via parent-directory traversal

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 4.4, Medium
Actively exploited?No
AffectedCopier-org copier (< 9.14.1)
Fixed in9.14.1
Type (CWE)CWE-22: CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

CVE-2026-34726 is a copier _subdirectory allows template root escape via parent-directory traversal in Copier-org copier. The fix is to upgrade to 9.14.1 and apply the runnable commands below.

What is CVE-2026-34726?

Copier is a library and CLI app for rendering project templates. Prior to version 9.14.1, Copier's _subdirectory setting is documented as the subdirectory to use as the template root. However, the current implementation accepts parent-directory traversal such as ..

In practical terms, a successful attacker gets unauthorized file reads or writes through path traversal. 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 Copier-org copier 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 copier   # Debian/Ubuntu
rpm -qa | grep -i copier   # RHEL/CentOS/Rocky

How to fix CVE-2026-34726

The primary fix is to upgrade to the patched build listed in the Fixed in row above (9.14.1). 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 copier
# Confirm the installed version meets or exceeds 9.14.1
dpkg -s copier | grep ^Version

Linux (RHEL / CentOS / Rocky)


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

Windows (PowerShell, admin)


winget upgrade --id 'Copierorg.copier' --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\copier-9.14.1.msi" -ArgumentList '/qn /norestart' -Wait

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


# Run as Administrator
$ErrorActionPreference = 'Stop'
$log = "$env:ProgramData\copier-Patch-CVE-2026-34726.log"
function Write-Log($msg) { "$(Get-Date -Format s)  $msg" | Tee-Object -FilePath $log -Append }

Write-Log "Starting CVE-2026-34726 remediation for Copier-org copier"

# 1. Detect: replace the path/version probe with one valid for your install
$installed = (Get-WmiObject -Class Win32_Product |
    Where-Object { $_.Name -like '*copier*' } |
    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]'9.14.1') {
    Write-Log "Already at fixed version $installed; no action needed."
    return
}

# 2. Backup configuration to a timestamped folder
$backup = "$env:ProgramData\copier-Backup-$(Get-Date -Format yyyyMMdd-HHmm)"
New-Item -ItemType Directory -Path $backup -Force | Out-Null
$src = "$env:ProgramFiles\Copier-org\copier"
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\copier-9.14.1.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 { $_.Name -like '*copier*' } |
    Select-Object -First 1 -ExpandProperty Version)
if ([version]$verify -ge [version]'9.14.1') {
    Write-Log "SUCCESS: now at $verify (>= 9.14.1)"
} 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/copier-patch-cve-2026-34726.log
log()  { echo "$(date -Iseconds)  $*" | tee -a "$LOG"; }

log "Starting CVE-2026-34726 remediation for Copier-org copier"

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

# 2. Backup config
BACKUP=/var/backups/copier-$(date +%Y%m%d-%H%M)
mkdir -p "$BACKUP"
for d in /etc/copier /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 copier
else
    sudo dnf upgrade --security -y copier
fi

# 4. Verify
if [ "$PKG_MGR" = apt ]; then
    NEW=$(dpkg-query -W -f='${Version}' copier)
else
    NEW=$(rpm -q --queryformat '%{VERSION}' copier)
fi
log "After upgrade: $NEW"
log "Done. Compare $NEW against 9.14.1 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.

Reject ../ and encoded variants at the reverse proxy

nginx:


location / {
    if ($request_uri ~* "(\.\./|%2e%2e/|%2e%2e%2f|%2f%2e%2e)") { return 403; }
    proxy_pass http://backend;
}

Apache mod_rewrite:


RewriteEngine On
RewriteCond %{REQUEST_URI} (\.\./|%2e%2e/|%2f%2e%2e) [NC]
RewriteRule .* - [F]

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 "copier"   # Debian/Ubuntu
rpm -qa | grep -i "copier"   # RHEL/CentOS/Rocky

Expected output: the package version should meet or exceed 9.14.1.

Re-run any vulnerability scanner you used previously and confirm the finding for CVE-2026-34726 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-2026-34726 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-2026-34726?

CVSS rates it 4.4 (Medium). Use that score to set your patch priority alongside the other items in your queue.

Do I have to take copier 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-2026-34726 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.*