raise an error when STARTTLS fails

Lib/smtplib.py

@@ -695,6 +695,11 @@ class SMTP:
             self.ehlo_resp = None
             self.esmtp_features = {}
             self.does_esmtp = 0
+        else:
+            # RFC 3207:
+            # 501 Syntax error (no parameters allowed)
+            # 454 TLS not available due to temporary reason
+            raise SMTPResponseException(resp, reply)
         return (resp, reply)
 
     def sendmail(self, from_addr, to_addrs, msg, mail_options=[],

Misc/NEWS

@@ -19,6 +19,9 @@ Core and Builtins
 Library
 -------
 
+- Fix TLS stripping vulnerability in smptlib, CVE-2016-0772.  Reported by Team
+  Oststrom
+
 - Issue #25939: On Windows open the cert store readonly in ssl.enum_certificates.
 
 - Issue #26012: Don't traverse into symlinks for ** pattern in