Werkzeug safe_join() allows Windows special device names

MEDIO 6.3CVE-2026-102598 · GHSA-g6x2-hccm-hh4m · PYSEC-2026-4312 · Published 29/09/2026 · updated 30/09/2026

In breve

CVE-2026-102598 è una vulnerabilità MEDIO (CVSS 6.3) che riguarda werkzeug.
Probabilità di sfruttamento nei prossimi 30 giorni (EPSS): 0.37%, superiore al 30.9% delle vulnerabilità.
Correzione: aggiornare a 3.1.9 o successiva.

Package	Affected versions	Patched versions
werkzeug PyPI	`< 3.1.9`	`3.1.9`

Description

Werkzeug's safe_join function allows Windows device names as filenames when they have an empty ADS marker on NTFS.

This was previously reported as https://github.com/pallets/werkzeug/security/advisories/GHSA-hgf8-39gv-g3f2, but the added filtering failed to account for the fact Windows allows special device names with an empty ADS marker, such as NUL:.

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 NTFS, and the requested path ends with a special device name, the file will be opened successfully, but reading will hang indefinitely.

NVD description

Werkzeug is a comprehensive WSGI web application library. Prior to 3.1.9, the safe_join function used by send_from_directory can allow a NUL: special-device path because safe_join checks the Windows device name without first removing an empty NTFS ADS marker. The trigger is that an application runs on Windows with NTFS and serves a user-specified path ending in a special device name such as NUL:. The attack mechanism is that a requested path ends in a Windows special device name with an empty ADS marker. The impact is that the special device opens successfully and the file read hangs indefinitely. This issue is fixed in version 3.1.9.