Werkzeug's safe_join function allows Windows device names as filenames if when preceded by other path segments.
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 that safe_join accepts paths with multiple segments, such as example/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 the requested path ends with a special device name, the file will be opened successfully, but reading will hang indefinitely.
{
"nvd_published_at": "2026-02-21T06:17:00Z",
"github_reviewed_at": "2026-02-19T20:32:45Z",
"github_reviewed": true,
"severity": "MODERATE",
"cwe_ids": [
"CWE-67"
]
}