# CVE-2025-66221: werkzeug — Fixed in 3.1.4

🐍 PyPI

Not in CISA KEV

# CVE-2025-66221 — werkzeug

[Fix: pallets/werkzeug@4b83337](https://github.com/pallets/werkzeug/commit/4b833376a45c323a189cd11d2362bcffdb1c0c13)

CVE-2025-66221 is a CWE-67 vulnerability in werkzeug. A fix is available for werkzeug — see the affected versions and patch details below.

Werkzeug safe\_join() allows Windows special device names

Also known as[GHSA-hgf8-39gv-g3f2](/vulnerability/GHSA-hgf8-39gv-g3f2)PYSEC-2026-2046

Published

Nov 29, 2025

Updated

Oct 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

## Exploitation Status

### No confirmed exploitation observed yet

-   CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
-   CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2025-66221.

## EPSS Exploitation Probability

[via FIRST.org ↗](https://www.first.org/epss/)

0.5%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs46th percentile — riskier than 46% of all scored CVEsHighest risk

0.00%0.35%0.70%1.05%0.0%0.5%May 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from [FIRST.org EPSS](https://www.first.org/epss/).

## Real-World Exposure

1 pkg affected

🐍`werkzeug`

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

## Description

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.

## Affected Packages

1 total 1 fixed

Ecosystem

Package

Vulnerable range

Fix

🐍PyPI

`werkzeug` 

all versions

3.1.4`pip install --upgrade 'werkzeug==3.1.4'`

## Affected Products

1 product · 1 configurations

Application

`werkzeug`palletsprojects

< 3.1.4

range

## Detection & mitigation playbook

Open-source dependency

1.  ### Detect
    
    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for werkzeug, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
    
2.  ### Fix
    
    Update werkzeug to 3.1.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-66221 is resolved across your whole dependency graph.
    
3.  ### Workarounds
    
    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
    

## Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

This vulnerability is not applicable to Red Hat products as it specifically affects applications running on Microsoft Windows operating systems. The flaw in Werkzeug's \`safe\_join\` function, which allows denial of service via Windows device names in path segments, does not manifest in Linux environments.

Workaround published by Red Hat

> Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.

Source: [Red Hat security advisory for CVE-2025-66221](https://access.redhat.com/security/cve/cve-2025-66221) (CC BY 4.0)

## Frequently Asked Questions

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

O3 Security · Impact-Aware SCA

### Is CVE-2025-66221 in your dependencies?

Find it across PyPI, including transitive dependencies.

[Scan my dependencies](/book-demo) [How O3 SCA works](/impact-aware-sca)