A flaw was found in the Python-requests package, where it is vulnerable to potentially leaking Proxy-Authorization headers to destination servers, specifically during redirects to an HTTPS origin. This is a product of how rebuild_proxies is used to recompute and reattach the Proxy-Authorization header to requests when redirected. This behavior only affects proxied requests when credentials are supplied in the URL user information component (for example, https://username:password@proxy:8080).