Werkzeug WSGI Improper Handling of Windows Device Names Vulnerability - CVE-2026-27199

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.