Improper Input Validation

Summary

Werkzeug is a comprehensive WSGI web application library. Browsers may allow "nameless" cookies that look like "=value" instead of "key=value". A vulnerable browser may allow a compromised application on an adjacent subdomain to exploit this to set a cookie like "=__Host-test=bad" for another subdomain. Werkzeug prior to 2.2.3 will parse the cookie "=__Host-test=bad" as "__Host-test=bad". If a Werkzeug application is running next to a vulnerable or malicious subdomain which sets such a cookie using a vulnerable browser, the Werkzeug application will see the bad cookie value but the valid cookie key.