CVE-2026-39844: NiceGUI Path Traversal Vulnerability

MODERATE
Advisory Identifier: GHSA-w8wv-vfpc-hw2w
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CVSS 5.9
CWE-22
Published
Advisory Summary

CVE-2026-39844 is a MODERATE (CVSS 5.9) path traversal vulnerability (CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')) affecting NiceGUI libraries in the PyPI ecosystem. The issue involves upload filename sanitization bypass via backslashes allows path traversal on windows. Remediation is available by upgrading affected packages to version 3.10.0.

CVE-2026-39844 at a Glance

CVE Identifier CVE-2026-39844
GHSA Identifier GHSA-w8wv-vfpc-hw2w
Severity MODERATE
CVSS v3.1 Base Score 5.9
CWE Classification CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
Affected Product NiceGUI
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 5.9

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N).

Attack Vector Network (AV:N)
Attack Complexity High (AC:H)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity High (I:H)
Availability None (A:N)

How to Fix CVE-2026-39844

To remediate this vulnerability, upgrade all affected NiceGUI dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

nicegui fixed: 3.10.0 
pip install nicegui>=3.10.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show nicegui

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-39844, inspect your installed packages using pip:

pip show nicegui

Analyze the terminal output to locate any version of NiceGUI in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 3.10.0), your application is affected.

Note: The pip show nicegui command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal'), where the product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory. In NiceGUI, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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.

CVE-2026-39844 Affected NiceGUI Versions

1 component(s)
Fixed Versions:
3.10.0
Product / Package Ecosystem Affected Versions Fixed Version
nicegui
pypi/nicegui
PyPI
< 3.10.0
3.10.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/zauberzeug/nicegui/security/advisories/GHSA-w8wv-vfpc-hw2w
NVD — CVE-2026-39844 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-39844
Security Fix Commit FIX COMMIT
https://github.com/zauberzeug/nicegui/commit/d38a702e3af2da5b0708f689be8d71413fc77056
NiceGUI Source Repository REPOSITORY
https://github.com/zauberzeug/nicegui
NiceGUI Release RELEASE
https://github.com/zauberzeug/nicegui/releases/tag/v3.10.0

Frequently Asked Questions

What is CVE-2026-39844?
CVE-2026-39844 is a security vulnerability in NiceGUI involving Upload filename sanitization bypass via backslashes allows path traversal on Windows under CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal').
Which NiceGUI versions are affected?
Affected versions include releases prior to 3.10.0 across affected packages: nicegui.
What version fixes CVE-2026-39844?
Upgrading to 3.10.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is nicegui affected?
Yes, nicegui is affected in releases prior to 3.10.0. Upgrade your dependency to 3.10.0 or later.
What is the CVSS score of CVE-2026-39844?
CVE-2026-39844 has a CVSS v3.1 base score of 5.9 (MODERATE) with vector string CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N.
What CWE is associated with CVE-2026-39844?
CVE-2026-39844 is classified under CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') (The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.).
How can I check PyPI dependencies for CVE-2026-39844?
Run the dependency tree command: 'pip show nicegui'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-w8wv-vfpc-hw2w",
  "modified" : "2026-04-09T14:28:46Z",
  "published" : "2026-04-08T15:04:13Z",
  "aliases" : [ "CVE-2026-39844" ],
  "summary" : "NiceGUI: Upload filename sanitization bypass via backslashes allows path traversal on Windows",
  "details" : "### Summary\n\nThe 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.\n\nApplications that construct file paths using `file.name` (a pattern demonstrated in NiceGUI's bundled examples) are vulnerable to arbitrary file write on Windows.\n\n### Details\n\nThe sanitization in `nicegui/elements/upload_files.py` uses:\n\n```python\nfilename = PurePosixPath(upload.filename or '').name\n```\n\n`PurePosixPath` treats backslashes as literal characters, not path separators:\n\n```python\n>>> PurePosixPath('..\\\\..\\\\secret\\\\evil.txt').name\n'..\\\\..\\\\secret\\\\evil.txt'  # Not stripped!\n```\n\nWhen 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.\n\n### Impact\n\nOn Windows deployments of NiceGUI applications that use `file.name` in path construction:\n\n- **Arbitrary file write** outside the intended upload directory\n- **Potential remote code execution** through overwriting application files or placing executables in known locations\n- **Data integrity loss** through overwriting existing files\n\nLinux and macOS are not affected, as they treat backslashes as literal filename characters.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "nicegui"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.10.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.9.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/zauberzeug/nicegui/security/advisories/GHSA-w8wv-vfpc-hw2w"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-39844"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/zauberzeug/nicegui/commit/d38a702e3af2da5b0708f689be8d71413fc77056"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/zauberzeug/nicegui"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/zauberzeug/nicegui/releases/tag/v3.10.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-22" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-08T15:04:13Z",
    "nvd_published_at" : "2026-04-08T21:16:59Z"
  }
}