CVE-2023-23934: Wrkzeug's incorrect parsing of nameless cookies leads to __Host- cookies bypass

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.

— Red Hat

Pallets Werkzeug could allow a remote attacker to bypass security restrictions, caused by improper input validation. By sending a specially-crafted request, an attacker could exploit this vulnerability to set a cookie like =Host-test=bad for another subdomain.

— IBM

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. The issue is fixed in Werkzeug 2.2.3.