CVE-2021-23393: Flask Unchained Project Flask Unchained

Medium severity, CVSS 5.4. EPSS: 0.7% chance of exploitation in the next 30 days.

This affects the package Flask-Unchained before 0.9.0. When using the the _validate_redirect_url function, it is possible to bypass URL validation and redirect a user to an arbitrary URL by providing multiple back slashes such as \\\evil.com/path. This vulnerability is only exploitable if an alternative WSGI server other than Werkzeug is used, or the default behaviour of Werkzeug is modified using 'autocorrect_location_header=False.

Affected products

• Flask Unchained Project Flask Unchained: before 0.9.0 (fixed in 0.9.0)

Published 2021-06-11. Last modified 2026-06-17.