When using urllib3's proxy support with `ProxyManager`, the `Proxy-Authorization` header is only sent to the configured proxy, as expected.

However, when sending HTTP requests *without* using urllib3's proxy support, it's possible to accidentally configure the `Proxy-Authorization` header even though it won't have any effect as the request is not using a forwarding proxy or a tunneling proxy. In those cases, urllib3 doesn't treat the `Proxy-Authorization` HTTP header as one carrying authentication material and thus doesn't strip the header on cross-origin redirects.

Because this is a highly unlikely scenario, we believe the severity of this vulnerability is low for almost all users. Out of an abundance of caution urllib3 will automatically strip the `Proxy-Authorization` header during cross-origin redirects to avoid the small chance that users are doing this on accident.

Users should use urllib3's proxy support or disable automatic redirects to achieve safe processing of the `Proxy-Authorization` header, but we still decided to strip the header by default in order to further protect users who aren't using the correct approach.

## Affected usages

We believe the number of usages affected by this advisory is low. It requires all of the following to be true to be exploited:

* Setting the `Proxy-Authorization` header without using urllib3's built-in proxy support.
* Not disabling HTTP redirects.
* Either not using an HTTPS origin server or for the proxy or target origin to redirect to a malicious origin.

## Remediation

* Using the `Proxy-Authorization` header with urllib3's `ProxyManager`.
* Disabling HTTP redirects using `redirects=False` when sending requests.
* Not using the `Proxy-Authorization` header.