The smtplib library in CPython (aka Python) before 2.7.12, 3.x before 3.4.5, and 3.5.x before 3.5.2 does not return an error when StartTLS fails, which might allow man-in-the-middle attackers to bypass the TLS protections by leveraging a network position between the client and the registry to block the StartTLS command, aka a “StartTLS stripping attack.”
lists.opensuse.org/opensuse-security-announce/2020-01/msg00040.html
rhn.redhat.com/errata/RHSA-2016-1626.html
rhn.redhat.com/errata/RHSA-2016-1627.html
rhn.redhat.com/errata/RHSA-2016-1628.html
rhn.redhat.com/errata/RHSA-2016-1629.html
rhn.redhat.com/errata/RHSA-2016-1630.html
www.openwall.com/lists/oss-security/2016/06/14/9
www.securityfocus.com/bid/91225
www.splunk.com/view/SP-CAAAPSV
www.splunk.com/view/SP-CAAAPUE
bugzilla.redhat.com/show_bug.cgi?id=1303647
docs.python.org/3.4/whatsnew/changelog.html#python-3-4-5
docs.python.org/3.5/whatsnew/changelog.html#python-3-5-2
hg.python.org/cpython/raw-file/v2.7.12/Misc/NEWS
hg.python.org/cpython/rev/b3ce713fb9be
hg.python.org/cpython/rev/d590114c2394
lists.debian.org/debian-lts-announce/2019/02/msg00011.html
security.gentoo.org/glsa/201701-18