What is the CVSS score of CVE-2026-39844?

CVE-2026-39844 has a CVSS 3.1 base score of 5.9 (Medium). CVSS vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N. EPSS exploitation probability: 0.1%.

Is there a patch available for CVE-2026-39844?

Yes, a patch is available for CVE-2026-39844. Update the affected software to the latest version immediately.

Python CVE-2026-39844

EUVD-2026-20610 MEDIUM

Path Traversal (CWE-22)

2026-04-08 https://github.com/zauberzeug/nicegui

GHSA-w8wv-vfpc-hw2w

Python Path Traversal Apple RCE Microsoft

5.9

CVSS 3.1 · GitHub Advisory

Severity by source

GitHub Advisory PRIMARY

5.9 MEDIUM

AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

Primary rating from GitHub Advisory · only source for this CVE.

CVSS VectorGitHub Advisory

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

Attack Vector

Network

Attack Complexity

High

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

High

Availability

None

Lifecycle Timeline

EUVD ID Assigned

Apr 08, 2026 - 15:16 euvd

EUVD-2026-20610

Analysis Generated

Apr 08, 2026 - 15:16 vuln.today

Patch released

Apr 08, 2026 - 15:16 nvd

Patch available

CVE Published

Apr 08, 2026 - 15:04 nvd

MEDIUM 5.9

Blast Radius

ecosystem impact

† from your stack dependencies † transitive graph · vuln.today resolves 4-path depth

4 pypi packages depend on nicegui (3 direct, 1 indirect)

Ecosystem-wide dependent count for version 3.10.0.

DescriptionGitHub Advisory

Summary

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:

python

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

PurePosixPath treats backslashes as literal characters, not path separators:

python

>>> 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.

Impact

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.

AnalysisAI

Path traversal via backslash bypass in NiceGUI file upload sanitization allows arbitrary file write on Windows systems. The vulnerability exploits a cross-platform path handling inconsistency where PurePosixPath fails to strip backslash-based path traversal sequences, enabling attackers to write files outside the intended upload directory when applications construct paths using the sanitized filename. Windows deployments are exclusively affected; potential remote code execution is possible if executables or application files can be overwritten. No public exploit code identified at time of analysis, though the vulnerability is confirmed in NiceGUI versions prior to 3.10.0.

Technical ContextAI

NiceGUI's filename sanitization in nicegui/elements/upload_files.py uses Python's pathlib.PurePosixPath to strip directory traversal sequences (CWE-22). PurePosixPath only recognizes forward slashes (/) as path separators per POSIX specification and treats backslashes as literal characters. On Windows, the subsequent file path construction using pathlib.Path interprets backslashes as directory separators, creating a semantic mismatch between the sanitization layer (POSIX model) and the filesystem layer (Windows model). This allows payloads like '..\..\secret\evil.txt' to bypass sanitization and resolve to arbitrary filesystem locations when processed by Windows Path operations. The vulnerability is specific to Windows because Unix-like systems (Linux, macOS) treat backslashes as valid filename characters, rendering the payload harmless.

RemediationAI

Upgrade NiceGUI to version 3.10.0 or later, which corrects the path sanitization logic. The upstream fix (commit d38a702e3af2da5b0708f689be8d71413fc77056) replaces PurePosixPath-based sanitization with platform-aware path handling. For applications unable to upgrade immediately, implement additional validation: use pathlib.Path(uploads_dir).resolve() to obtain the canonical absolute path of the upload directory, then verify that any constructed file path (using Path.resolve()) remains within that directory before writing. Reference: https://github.com/zauberzeug/nicegui/security/advisories/GHSA-w8wv-vfpc-hw2w and https://github.com/zauberzeug/nicegui/releases/tag/v3.10.0.