CVE-2025-66221: PYSEC-2026-2046 - Werkzeug safe_join() allows Windows special device names

CVE-2025-66221 · Severity: medium · CVSS 5.3 · Published 2025-11-29

Werkzeug is a comprehensive WSGI web application library. Prior to version 3.1.4, Werkzeug's safe_join function allows path segments with Windows device names. On Windows, there are special device names such as CON, AUX, etc that are implicitly present and readable in every directory. send_from_directory uses safe_join to safely serve files at user-specified paths under a directory. If the application is running on Windows, and the requested path ends with a special device name, the file will be opened successfully, but reading will hang indefinitely. This issue has been patched in version 3.1.4.

Technologies: Microsoft Windows, werkzeug (PyPI). Vendors: Microsoft, PyPI.

Quick answers

Is CVE-2025-66221 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2025-66221 being exploited in the wild.
How severe is CVE-2025-66221?
Medium severity, with a CVSS score of 5.3, published 29 November 2025.
Which products and versions are affected?
werkzeug (PyPI) ECOSYSTEM:introduced=0,fixed=3.1.4.
Is there a fix for CVE-2025-66221?
Yes. werkzeug (PyPI): upgrade to 3.1.4 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
Am I affected by CVE-2025-66221?
Check whether you run Microsoft Windows, Palletsprojects Werkzeug or werkzeug (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

  • werkzeug (PyPI): upgrade to 3.1.4 or later
    pip install "werkzeug>=3.1.4"

Affected products

  • Microsoft Windows
  • Palletsprojects Werkzeug
  • werkzeug (PyPI) ECOSYSTEM:introduced=0,fixed=3.1.4

Identifiers

  • CVE-2025-66221
  • GHSA-hgf8-39gv-g3f2
  • PYSEC-2026-2046

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