`Cookie` HTTP header isn't stripped on cross-origin redirects

urllib3 doesn't treat the Cookie HTTP header special or provide any helpers for managing cookies over HTTP, that is the responsibility of the user. However, it is possible for a user to specify a Cookie header and unknowingly leak information via HTTP redirects to a different origin if that user doesn't disable redirects explicitly.

Users must handle redirects themselves instead of relying on urllib3's automatic redirects to achieve safe processing of the Cookie header, thus we 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:

• Using an affected version of urllib3 (patched in v1.26.17 and v2.0.6)
• Using the Cookie header on requests, which is mostly typical for impersonating a browser.
• Not disabling HTTP redirects
• Either not using HTTPS or for the origin server to redirect to a malicious origin.

Remediation

• Upgrading to at least urllib3 v1.26.17 or v2.0.6
• Disabling HTTP redirects using redirects=False when sending requests.
• Not using the Cookie header.