pip before 1.3 uses HTTP to retrieve packages from the PyPI repository, and does not perform integrity checks on package contents, which allows man-in-the-middle attackers to execute arbitrary code via a crafted response to a "pip install" operation.
https://github.com/pypa/pip/pull/791/files
https://github.com/pypa/pip/issues/425
https://bugzilla.redhat.com/show_bug.cgi?id=968059