CVE-2024-37891

urllib3 is a user-friendly HTTP client library for Python. When using urllib3s proxy support with ProxyManager, the Proxy-Authorization header is only sent to the configured proxy, as expected. However, when sending HTTP requests without using urllib3s proxy support, its possible to accidentally configure the Proxy-Authorization header even though it wont have any effect as the request is not using a forwarding proxy or a tunneling proxy. In those cases, urllib3 doesnt treat the Proxy-Authorization HTTP header as one carrying authentication material and thus doesnt 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 urllib3s 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 arent 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 urllib3s 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 urllib3s ProxyManager, disable HTTP redirects using redirects=False when sending requests, or not user the Proxy-Authorization header as mitigations.

Weakness

The product does not properly transfer a resource/behavior to another sphere, or improperly imports a resource/behavior from another sphere, in a manner that provides unintended control over that resource.

Affected Software

References

• https://github.com/urllib3/urllib3/commit/accff72ecc2f6cf5a76d9570198a93ac7c90270e
• https://github.com/urllib3/urllib3/security/advisories/GHSA-34jh-p97f-mpxf