Werkzeug safe_join() allows Windows special device names
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.
describes (1)
Related CVEs.
| ID | severity | exploitable | ||
|---|---|---|---|---|
| CVE-2026-27199US | medium5.3 | Exploit | ||
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 segmen… | ||||
references
External citations — patches, advisories, analysis.
- https://github.com/pallets/werkzeugPACKAGE
- https://github.com/pallets/werkzeug/commit/f407712fdc60a09c2b3f4fe7db557703e5d9338dWEB
- https://github.com/pallets/werkzeug/releases/tag/3.1.6WEB
- https://github.com/pallets/werkzeug/security/advisories/GHSA-29vq-49wr-vm6xWEB
- https://nvd.nist.gov/vuln/detail/CVE-2026-27199ADVISORY
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