CVE-2026-27199: Werkzeug safe_join() allows Windows special device names
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.
Affected Software
Remediation
Event History
Frequently Asked Questions
What is the severity of CVE-2026-27199?
CVE-2026-27199 has a moderate severity level, primarily affecting Windows environments.
How do I fix CVE-2026-27199?
To fix CVE-2026-27199, upgrade Werkzeug to version 3.1.6 or higher.
Which versions of Werkzeug are affected by CVE-2026-27199?
Werkzeug versions 3.1.5 and below are affected by CVE-2026-27199.
What functionality is impacted by CVE-2026-27199?
CVE-2026-27199 impacts the safe_join function, allowing Windows special device names as filenames.
Is there a known exploit for CVE-2026-27199?
While no specific exploit is documented, the vulnerability can potentially lead to security risks if unvalidated input is processed.