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

Severity: medium (CVSS 5.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-11-29 · updated 2026-10-07
Weakness: CWE-67

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 products

pallets werkzeug: before 3.1.4 — fixed in 3.1.4
Pypi Werkzeug: before 3.1.4; 0.1, 0.10, 0.10.1, 0.10.2, 0.10.3, 0.10.4 (+95) — fixed in 3.1.4

How to fix

Upgrade to a fixed version: pallets werkzeug 3.1.4; Pypi Werkzeug 3.1.4. Check the vendor advisory for mitigations.

References

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