GitHub Advisoryadvisory · PyPI
GHSA-87hc-h4r5-73f7 Werkzeug safe_join() allows Windows special device names with compound extensions
MEDIUM
Official page ↗
- Aliases
- CVE-2026-21860
- Ecosystem
- PyPI
- Packages
- Werkzeug
- Severity
- Moderate
- CVSS vector
- CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- Weaknesses
- CWE-67
- Reviewed
- 2026-01-08
Affected packages
• PyPI Werkzeug: < 3.1.5 → fixed in 3.1.5
Description
Werkzeug's `safe_join` 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 was previously reported as https://github.com/pallets/werkzeug/security/advisories/GHSA-hgf8-39gv-g3f2, but the fix failed to account for compound extensions such as `CON.txt.html` or trailing spaces. It also missed some additional special names.
`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.
Related
Alias · CVE (1)
- CVE-2026-21860 Werkzeug is a comprehensive WSGI web application library. vulnerability
Weakness · CWE (1)
- CWE-67 Improper Handling of Windows Device Names weakness