CVE-2026-39844: NiceGUI Path Traversal Vulnerability

CVE-2026-39844 Overview

CVE-2026-39844 is a path traversal vulnerability in NiceGUI, a Python-based UI framework. Prior to version 3.10.0, the application uses PurePosixPath for path sanitization, which only recognizes forward slashes (/) as path separators. This design flaw allows an attacker to bypass path sanitization on Windows systems by using backslashes (\) in upload filenames, leading to arbitrary file write capabilities.

Critical Impact

Applications that construct file paths using file.name (a pattern demonstrated in NiceGUI's bundled examples) are vulnerable to arbitrary file write on Windows systems, potentially enabling code execution or system compromise.

Affected Products

• NiceGUI versions prior to 3.10.0
• Windows-based deployments using NiceGUI file upload functionality
• Applications following NiceGUI bundled example patterns for file handling

Discovery Timeline

• 2026-04-08 - CVE-2026-39844 published to NVD
• 2026-04-08 - Last updated in NVD database

Technical Details for CVE-2026-39844

Vulnerability Analysis

This vulnerability stems from a fundamental incompatibility between path handling libraries and operating system conventions. When NiceGUI processes file uploads, it relies on Python's PurePosixPath class for path sanitization. This class is designed for POSIX-compliant systems (Linux, macOS) and only interprets forward slashes (/) as directory separators.

On Windows systems, however, backslashes (\) are the native path separators. An attacker can craft a malicious filename containing backslash characters that appear harmless to the POSIX path sanitization logic but are interpreted as directory traversal sequences by the Windows file system when the file is written.

The vulnerability is classified as CWE-22 (Improper Limitation of a Pathname to a Restricted Directory), commonly known as path traversal. Exploitation requires network access and the ability to submit crafted filenames through the file upload functionality. While the attack complexity is considered high due to the Windows-specific requirement, successful exploitation results in high integrity impact through arbitrary file writes.

Root Cause

The root cause is the use of PurePosixPath for filename sanitization across all platforms, including Windows. This POSIX-specific class does not recognize Windows path separators (\), allowing malicious backslash-containing filenames to bypass directory traversal protections. When the application subsequently writes the file using the unsanitized filename, the Windows file system interprets the backslashes as path separators, enabling writes outside the intended directory.

Attack Vector

An attacker can exploit this vulnerability by uploading a file with a specially crafted filename containing backslash sequences. For example, a filename such as ..\..\..\..\sensitive_path\malicious_file.py would be treated as a simple filename by PurePosixPath but would traverse directories when processed by Windows. This allows attackers to write arbitrary files to any location accessible by the application's process, potentially overwriting critical system files or planting malicious code.

The attack is network-accessible and requires no authentication or user interaction. Applications following the example patterns in NiceGUI's documentation are particularly vulnerable as they directly use file.name when constructing output paths. For detailed technical analysis, refer to the GitHub Security Advisory GHSA-w8wv-vfpc-hw2w.

Detection Methods for CVE-2026-39844

Indicators of Compromise

• Uploaded files appearing in unexpected directories outside the designated upload folder
• File paths containing backslash sequences (\) in upload logs on Windows systems
• Unexpected file modifications in application directories or system paths
• Presence of files with traversal patterns in filename metadata

Detection Strategies

• Monitor file system activity for writes outside designated upload directories
• Implement logging of all uploaded filenames and inspect for backslash characters on Windows deployments
• Review application logs for filenames containing patterns like ..\ or multiple consecutive backslashes
• Use file integrity monitoring (FIM) to detect unauthorized changes to critical application files

Monitoring Recommendations

• Deploy endpoint detection and response (EDR) solutions to monitor file creation events outside expected paths
• Configure SentinelOne Singularity Platform to detect suspicious file write operations from web application processes
• Implement alerting for any file operations where the destination path differs significantly from the configured upload directory
• Enable audit logging for file system access on Windows servers running NiceGUI applications

How to Mitigate CVE-2026-39844

Immediate Actions Required

• Upgrade NiceGUI to version 3.10.0 or later immediately
• Audit existing codebases for patterns that directly use file.name when constructing file paths
• Implement server-side filename sanitization that normalizes both forward and backslash characters
• Review recently uploaded files for evidence of path traversal exploitation

Patch Information

The vulnerability has been fixed in NiceGUI version 3.10.0. The security patch addresses the path sanitization bypass by properly handling Windows-style path separators. Organizations should update to the patched version as soon as possible.

For technical details on the fix, refer to the GitHub Commit Update and the GitHub Release v3.10.0.

Workarounds

• Replace PurePosixPath with pathlib.Path for cross-platform path handling in custom upload handlers
• Implement explicit filename sanitization that removes or replaces both / and \ characters before constructing file paths
• Use os.path.basename() to extract only the filename portion, discarding any path components
• Restrict file uploads to Linux-based servers where this specific bypass is not applicable

# Configuration example
# Upgrade NiceGUI to the patched version
pip install --upgrade nicegui>=3.10.0

# Verify installed version
pip show nicegui | grep Version