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

Severity: low (CVSS 3.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2023-02-14 · updated 2026-06-17
Weakness: CWE-20

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.

Affected products

pallets werkzeug: before 2.2.3 — fixed in 2.2.3
Pypi Werkzeug: before 2.2.3; 0.1, 0.10, 0.10.1, 0.10.2, 0.10.3, 0.10.4 (+74) — fixed in 2.2.3

How to fix

Upgrade to a fixed version: pallets werkzeug 2.2.3; Pypi Werkzeug 2.2.3. Check the vendor advisory for mitigations.

References

Get alerted when CVE-2023-23934 or the next vulnerability affects your stack