Werkzeug is a comprehensive WSGI web application library. Prior to 3.1.9, the safejoin function used by sendfromdirectory can allow a NUL: special-device path because safejoin 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.

Werkzeug is a comprehensive WSGI web application library. Prior to version 3.1.5, Werkzeug's safejoin function allows path segments with Windows device names that have file extensions or trailing spaces. On Windows, there are special device names such as CON, AUX, etc that are implicitly present and readable in every directory. Windows still accepts them with any file extension, such as CON.txt, or trailing spaces such as CON. This issue has been patched in version 3.1.5.