urllib3 allows a low‑level redirect mechanism to forward sensitive HTTP headers, such as authentication tokens or cookies, to a third‑party origin when the call uses ProxyManager.connection_from_url() with assert_same_host disabled. This results in a confidentiality loss, enabling an attacker to read headers the application intended to keep confined, and is classified as CWE‑200 and CWE‑201.

All installations of urllib3 from version 1.23 through versions before 2.7.0 are affected. The issue resides in the library’s ProxyManager component that follows low‑level redirects.

The CVSS score of 8.2 marks the vulnerability as high severity, and the EPSS score of <1% indicates a very low exploitation probability. Exploitation requires an application that uses the vulnerable redirect path and permits assert_same_host=False, which is a non‑common but feasible configuration in many Python web clients. No public exploit has been reported, but the potential to leak sensitive headers makes it a compelling target for attackers with access to the redirecting service. The vulnerability is not listed in the CISA KEV catalog.