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.\