CVE-2026-39844

The upload filename sanitization introduced in GHSA-9ffm-fxg3-xrhh uses PurePosixPath(filename).name to strip path components. 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.

Details

The sanitization in nicegui/elements/upload_files.py uses:

filename = PurePosixPath(upload.filename or '').name

PurePosixPath treats backslashes as literal characters, not path separators:

>>> PurePosixPath('..\\..\\secret\\evil.txt').name
'..\\..\\secret\\evil.txt'  # Not stripped!

When this filename is used in a path operation on Windows (e.g., Path('uploads') / file.name), Windows Path interprets backslashes as directory separators, resolving the path outside the intended directory.

On Windows deployments of NiceGUI applications that use file.name in path construction:

• Arbitrary file write outside the intended upload directory
• Potential remote code execution through overwriting application files or placing executables in known locations
• Data integrity loss through overwriting existing files

Linux and macOS are not affected, as they treat backslashes as literal filename characters.

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

CVE-2026-39844 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.10.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-39844? CVE-2026-39844 is a medium-severity path traversal vulnerability in nicegui (pip), affecting versions <= 3.9.0. It is fixed in 3.10.0. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is CVE-2026-39844? CVE-2026-39844 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of nicegui are affected by CVE-2026-39844? nicegui (pip) versions <= 3.9.0 is affected.
4. Is there a fix for CVE-2026-39844? Yes. CVE-2026-39844 is fixed in 3.10.0. Upgrade to this version or later.
5. Is CVE-2026-39844 exploitable, and should I be worried? Whether CVE-2026-39844 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-39844 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-39844? Upgrade nicegui to 3.10.0 or later.