Wrkzeug's incorrect parsing of nameless cookies leads to __Host- cookies bypass

Vulnerability Description

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.

References

• https://github.com/pallets/werkzeug/security/advisories/GHSA-px8h-6qxv-m22q
• https://github.com/pallets/werkzeug/commit/cf275f42acad1b5950c50ffe8ef58fe62cdce028
• https://github.com/pallets/werkzeug/releases/tag/2.2.3
• https://www.debian.org/security/2023/dsa-5470
• https://security.netapp.com/advisory/ntap-20230818-0003/