The urlopen function in pym/portage/util/_urlopen.py in Gentoo Portage 2.1.12, when using HTTPS, does not verify X.509 certificates from SSL servers, which allows man-in-the-middle attackers to spoof servers and modify binary package lists via a crafted certificate.
https://security.gentoo.org/glsa/201507-16
https://exchange.xforce.ibmcloud.com/vulnerabilities/84315
https://bugs.gentoo.org/show_bug.cgi?id=469888