CVE-2026-27199: 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: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-02-21 · updated 2026-06-17
Weakness: CWE-67

Description

Werkzeug is a comprehensive WSGI web application library. Versions 3.1.5 and below, the safe_join function allows Windows device names as filenames if preceded by other path segments. This was previously reported as GHSA-hgf8-39gv-g3f2, but the added filtering failed to account for the fact that safe_join accepts paths with multiple segments, such as example/NUL. The function 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 fixed in version 3.1.6.

Affected products

pallets werkzeug: before 3.1.6 — fixed in 3.1.6
Pypi Werkzeug: before 3.1.6; 0.1, 0.10, 0.10.1, 0.10.2, 0.10.3, 0.10.4 (+97) — fixed in 3.1.6

How to fix

Upgrade to a fixed version: pallets werkzeug 3.1.6; Pypi Werkzeug 3.1.6. Check the vendor advisory for mitigations.

References

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