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

> Attackers can write arbitrary files on Windows systems by bypassing path sanitization in NiceGUI upload filenames using backslashes.

- **Severity:** Medium 5.9 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 8 Apr 2026
- **Updated:** 9 Apr 2026
- **Also known as:** GHSA-w8wv-vfpc-hw2w, PYSEC-2026-2234

## What is CVE-2026-39844?

CVE-2026-39844 is a medium-severity path traversal vulnerability in zauberzeug nicegui, published on 8 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-22 (Improper Limitation of a Pathname to a Restricted Directory)

## Which versions are affected by CVE-2026-39844?

3 open source packages ship an affected version, including nicegui. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `nicegui` | PyPI | All versions | 3.10.0 |
| `nicegui` | PyPI | All versions | 3.10.0 |
| `nicegui` | PyPI | All versions | 3.10.0 |

Products: Zauberzeug Nicegui.

## How do I fix CVE-2026-39844?

Upgrade nicegui to 3.10.0 or later.

## Is CVE-2026-39844 being exploited?

There is no known exploitation of CVE-2026-39844 in the wild and no public exploit code.

## Other vulnerabilities in zauberzeug nicegui

- [CVE-2026-25732](https://insights.sbomify.com/CVE-2026-25732/): NiceGUI's Path Traversal via Unsanitized FileUpload.name Enables Arbitrary File Write (high 7.5)
- [CVE-2025-66645](https://insights.sbomify.com/CVE-2025-66645/): NiceGUI Path Traversal Vulnerability in app.add_media_files() Allows Arbitrary File Reading (high 7.5)
- [CVE-2026-21873](https://insights.sbomify.com/CVE-2026-21873/): Zero-click XSS in all NiceGUI apps which uses `ui.sub_pages` (high 7.2)
- [CVE-2026-33332](https://insights.sbomify.com/CVE-2026-33332/): NiceGUI's unvalidated chunk size parameter in media routes can cause memory exhaustion (medium 6.9)
- [CVE-2026-27156](https://insights.sbomify.com/CVE-2026-27156/): NiceGUI has XSS via Code Injection (medium 6.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-39844 and fix it if it is affected.

What it is: CVE-2026-39844 is a medium-severity path traversal vulnerability in zauberzeug nicegui, published on 8 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-39844 in the wild and no public exploit code.

Affected packages and fixed versions:
- nicegui (PyPI): affected all versions; fixed in 3.10.0
- nicegui (PyPI): affected all versions; fixed in 3.10.0
- nicegui (PyPI): affected all versions; fixed in 3.10.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-39844/ (Markdown: https://insights.sbomify.com/CVE-2026-39844/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-39844
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-39844
- OSV: https://osv.dev/vulnerability/CVE-2026-39844

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-39844/
Summaries are generated and may contain mistakes; the official description above is the record.