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 <= 2.2.2 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.
Browsers may allow "nameless" cookies that look like
=valueinstead ofkey=value. A vulnerable browser may allow a compromised application on an adjacent subdomain to exploit this to set a cookie like=__Host-test=badfor another subdomain.Werkzeug <= 2.2.2 will parse the cookie
=__Host-test=badas__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.