CVE-2024-37891

urllib3 is a user-friendly HTTP client library for Python. 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. We believe the number
of usages affected by this advisory is low. It requires all of the
following to be true to be exploited: 1. Setting the Proxy-Authorization
header without using urllib3’s built-in proxy support. 2. Not disabling
HTTP redirects. 3. Either not using an HTTPS origin server or for the proxy
or target origin to redirect to a malicious origin. Users are advised to
update to either version 1.26.19 or version 2.2.2. Users unable to upgrade
may use the Proxy-Authorization header with urllib3’s ProxyManager,
disable HTTP redirects using redirects=False when sending requests, or
not user the Proxy-Authorization header as mitigations.

Notes