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.