The resolve_redirects function in sessions.py in requests 2.1.0 through 2.5.3 allows remote attackers to conduct session fixation attacks via a cookie without a host value in a redirect.

Research is free — Hunters explains how the bug works, the root-cause code pattern, how the fix addresses it, and how to test whether a target is affected, in chat. Investigate & write exploit is a paid run — the engine reads the advisory and fix commits, then builds and validates a working proof-of-concept exploit with reproduction steps.