HowToFixMe
● Medium · CVSS 5.9

How to Fix CVE-2026-39844: CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

By Sai Kiran Pandrala · reviewed by Sai Kiran Pandrala, Editor

Last verified: 2026-05-25

⚡ At a glance
SeverityCVSS 5.9, Medium
Actively exploited?No
AffectedZauberzeug nicegui (< 3.10.0)
Fixed in3.10.0
Type (CWE)CWE-22: CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

Exploitation status

CVE-2026-39844 is not currently on the CISA Known Exploited Vulnerabilities (KEV) catalog, so there is no U.S.-government-confirmed in-the-wild exploitation on record for it. Take that with caution, because KEV entries often appear well after attacks begin, so patch on severity rather than holding out for a listing.

Public exploit availability: the references currently cite no public exploit or Metasploit module. That is not evidence of safety, since private exploit code may exist, so do not treat it as low risk for that reason alone.

Authoritative references:

CVE-2026-39844 is a cwe-22: improper limitation of a pathname to a restricted directory ('path traversal') in Zauberzeug nicegui. The fix is to upgrade to 3.10.0 and apply the runnable commands below.

What is CVE-2026-39844?

NiceGUI is a Python-based UI framework. Prior to 3.10.0, Since PurePosixPath only recognizes forward slashes (/) as path separators, an attacker can bypass this sanitization on Windows by using backslashes (\) in the upload filename. Applications that construct file paths using file.name (a pattern demonstrated in NiceGUI's bundled examples) are vulnerable to arbitrary file write on Windows.

In practical terms, a successful attacker gets a security bypass on the affected component. 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.

What you'll see

You are affected if you run Zauberzeug nicegui 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 nicegui   # Debian/Ubuntu
rpm -qa | grep -i nicegui   # RHEL/CentOS/Rocky

How to fix CVE-2026-39844

The primary fix is to upgrade to the patched build listed in the Fixed in row above (3.10.0). 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 nicegui
# Confirm the installed version meets or exceeds 3.10.0
dpkg -s nicegui | grep ^Version

Linux (RHEL / CentOS / Rocky)

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

Windows (PowerShell, admin)

winget upgrade --id 'Zauberzeug.nicegui' --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\nicegui-3.10.0.msi" -ArgumentList '/qn /norestart' -Wait

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

# Run as Administrator
$ErrorActionPreference = 'Stop'
$log = "$env:ProgramData\nicegui-Patch-CVE-2026-39844.log"
function Write-Log($msg) { "$(Get-Date -Format s)  $msg" | Tee-Object -FilePath $log -Append }

Write-Log "Starting CVE-2026-39844 remediation for Zauberzeug nicegui"

# 1. Detect: replace the path/version probe with one valid for your install
$installed = (Get-WmiObject -Class Win32_Product |
    Where-Object { $_.Name -like '*nicegui*' } |
    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]'3.10.0') {
    Write-Log "Already at fixed version $installed; no action needed."
    return
}

# 2. Backup configuration to a timestamped folder
$backup = "$env:ProgramData\nicegui-Backup-$(Get-Date -Format yyyyMMdd-HHmm)"
New-Item -ItemType Directory -Path $backup -Force | Out-Null
$src = "$env:ProgramFiles\Zauberzeug\nicegui"
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\nicegui-3.10.0.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 { $_.Name -like '*nicegui*' } |
    Select-Object -First 1 -ExpandProperty Version)
if ([version]$verify -ge [version]'3.10.0') {
    Write-Log "SUCCESS: now at $verify (>= 3.10.0)"
} 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/nicegui-patch-cve-2026-39844.log
log()  { echo "$(date -Iseconds)  $*" | tee -a "$LOG"; }

log "Starting CVE-2026-39844 remediation for Zauberzeug nicegui"

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

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

# 4. Verify
if [ "$PKG_MGR" = apt ]; then
    NEW=$(dpkg-query -W -f='${Version}' nicegui)
else
    NEW=$(rpm -q --queryformat '%{VERSION}' nicegui)
fi
log "After upgrade: $NEW"
log "Done. Compare $NEW against 3.10.0 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/zauberzeug/nicegui/security/advisories/GHSA-w8wv-vfpc-hw2w
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/zauberzeug/nicegui/security/advisories/GHSA-w8wv-vfpc-hw2w
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>

The repair

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

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

Expected output: the package version should meet or exceed 3.10.0.

Re-run any vulnerability scanner you used previously and confirm the finding for CVE-2026-39844 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-2026-39844 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-2026-39844?

CVSS rates it 5.9 (Medium). Use that score to set your patch priority alongside the other items in your queue.

Do I have to take nicegui 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-2026-39844 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.

Additional nearby issues sensible to fix in the same maintenance window:

References

Written by Sai Kiran Pandrala