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.

References

• GHSA-px8h-6qxv-m22q
• https://nvd.nist.gov/vuln/detail/CVE-2023-23934
• pallets/werkzeug@cf275f4
• 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/