** backport r60015 from trunk.

- Issue829951: In the smtplib module, SMTP.starttls() now complies with
  RFC 3207 and forgets any knowledge obtained from the server not obtained
  from the TLS negotiation itself.  Patch contributed by Bill Fenner.

1 files changed, 8 insertions, 0 deletions

diff --git a/Lib/smtplib.py b/Lib/smtplib.py
index 9c8c4fa48c..1b9739c81a 100755
--- a/Lib/smtplib.py
+++ b/Lib/smtplib.py
@@ -605,6 +605,14 @@ class SMTP:
             sslobj = socket.ssl(self.sock, keyfile, certfile)
             self.sock = SSLFakeSocket(self.sock, sslobj)
             self.file = SSLFakeFile(sslobj)
+            # RFC 3207:
+            # The client MUST discard any knowledge obtained from
+            # the server, such as the list of SMTP service extensions,
+            # which was not obtained from the TLS negotiation itself.
+            self.helo_resp = None
+            self.ehlo_resp = None
+            self.esmtp_features = {}
+            self.does_esmtp = 0
         return (resp, reply)
 
     def sendmail(self, from_addr, to_addrs, msg, mail_options=[],