# CVE-2026-39844: Zauberzeug NiceGUI path traversal in file upload on Windows

Source: https://junglewise.ai/threats/cve-2026-39844-zauberzeug-nicegui-path-traversal-in-file-upload-on-windows  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-08. Updated: 2026-07-24  
License: CC BY 4.0 with a link back

CVE-2026-39844 · Severity: medium · CVSS 5.9 · Published 2026-04-08

NiceGUI is a Python framework used to build web-based user interfaces. A security flaw in how the framework handles file uploads on Windows systems allows an attacker to bypass safety checks by using backslashes in filenames. This could allow a remote attacker to write or overwrite files anywhere on the server's hard drive, potentially leading to a complete system takeover or data loss.

Technologies: [Microsoft Windows](https://junglewise.ai/threats/technologies/windows), [nicegui (PyPI)](https://junglewise.ai/threats/technologies/nicegui). Vendors: [Microsoft](https://junglewise.ai/threats/vendors/microsoft), [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2026-39844 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-39844 being exploited in the wild.

**How severe is CVE-2026-39844?** Medium severity, with a CVSS score of 5.9, published 8 April 2026.

**Which products and versions are affected?** nicegui (PyPI) ECOSYSTEM:introduced=0,fixed=3.10.0.

**Is there a fix for CVE-2026-39844?** Yes. nicegui (PyPI): upgrade to 3.10.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-39844 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-39844?** Check whether you run Microsoft Windows or nicegui (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- nicegui (PyPI): upgrade to 3.10.0 or later

  ```sh
  pip install "nicegui>=3.10.0"
  ```


## Technical details

A path traversal vulnerability exists in NiceGUI's file upload component when running on Windows. The framework used 'PurePosixPath' to sanitize filenames, which only recognizes forward slashes (/) as directory separators. On Windows, backslashes (\) are also valid separators; an attacker can provide a filename containing backslashes (e.g., '..\..\shell.exe') to bypass the 'PurePosixPath.name' filter. If the application uses the resulting 'file.name' to construct a storage path, it can result in arbitrary file write outside the intended directory. This issue is fixed in version 3.10.0 by explicitly splitting on both slash types.

## Affected products

- zauberzeug NiceGUI < 3.10.0

## Identifiers

- CVE-2026-39844
- GHSA-w8wv-vfpc-hw2w
- PYSEC-2026-2234

## Timeline

- 2026-04-07: patched: Fixed in version 3.10.0
- 2026-04-07: advisory: GitHub Security Advisory GHSA-w8wv-vfpc-hw2w published
- 2026-04-08: disclosed: CVE-2026-39844 published to NVD

## References

- [https://github.com/zauberzeug/nicegui/commit/d38a702e3af2da5b0708f689be8d71413fc77056](https://github.com/zauberzeug/nicegui/commit/d38a702e3af2da5b0708f689be8d71413fc77056)
- [https://github.com/zauberzeug/nicegui/releases/tag/v3.10.0](https://github.com/zauberzeug/nicegui/releases/tag/v3.10.0)
- [https://github.com/zauberzeug/nicegui/security/advisories/GHSA-w8wv-vfpc-hw2w](https://github.com/zauberzeug/nicegui/security/advisories/GHSA-w8wv-vfpc-hw2w)

## Related threats

- [Windows BitLocker heap buffer overflow with physical attack](https://junglewise.ai/threats/cve-2026-97015-heap-based-buffer-overflow-in-windows-bitlocker-allows-an)
- [Microsoft Windows Shell type confusion privilege elevation](https://junglewise.ai/threats/cve-2026-97013-access-of-resource-using-incompatible-type-type-confusion-in)
- [Microsoft Windows VMSwitch out-of-bounds read privilege escalation](https://junglewise.ai/threats/cve-2026-97002-out-of-bounds-read-in-windows-vmswitch-allows-an-authorized)
- [Microsoft Windows USM Communication Manager null pointer dereference](https://junglewise.ai/threats/cve-2026-97000-null-pointer-dereference-in-windows-usm-communication-manager)
- [Microsoft Windows Winsock use after free privilege escalation](https://junglewise.ai/threats/cve-2026-96994-use-after-free-in-winsock-allows-an-authorized-attacker-to)