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.
https://warehouse.python.org/project/requests/2.6.0/
https://github.com/kennethreitz/requests/commit/3bd8afbff29e50b38f889b2f688785a669b9aafc
http://www.ubuntu.com/usn/USN-2531-1
http://www.openwall.com/lists/oss-security/2015/03/15/1
http://www.openwall.com/lists/oss-security/2015/03/14/4
http://www.mandriva.com/security/advisories?name=MDVSA-2015:133
http://lists.fedoraproject.org/pipermail/package-announce/2015-March/153594.html