[git-multimail] smtplib, check certificate

[Simon P <simon.git@xxxxxxxxxx>]

Hi,

It seems that smtplib doesn't check if a certificate is valid (signed by
a trusted CA).

For my personal usage, I patched the starttls code in git-multimail:
only for starttls with smtplib.

This patch is inspired from

https://github.com/graingert/secure-smtplib/blob/master/src/secure_smtplib/__init__.py

It could be easy to add support cert check in for smtps (see
secure_smtplib).

This patch was tested only on git-multimail (v1.2)

It introduces two new options:
  - multimailhook.smtpcheckcert (default false)
  - multimailhook.smtpcacerts (default
                               /etc/ssl/certs/ca-certificates.crt)

Best regards,
Simon P.

diff --git a/git-multimail/git_multimail.py b/git-multimail/git_multimail.py
index fae5c91..b49ed9d 100755
--- a/git-multimail/git_multimail.py
+++ b/git-multimail/git_multimail.py
@@ -57,6 +57,7 @@ import subprocess
 import shlex
 import optparse
 import smtplib
+import ssl
 import time
 import cgi
 
@@ -1945,6 +1946,7 @@ class SMTPMailer(Mailer):
                  smtpservertimeout=10.0, smtpserverdebuglevel=0,
                  smtpencryption='none',
                  smtpuser='', smtppass='',
+                 smtpcacerts='/etc/ssl/certs/ca-certificates.crt',smtpcheckcert=False
                  ):
         if not envelopesender:
             sys.stderr.write(
@@ -1974,13 +1976,43 @@ class SMTPMailer(Mailer):
             if self.security == 'none':
                 self.smtp = call(smtplib.SMTP, self.smtpserver, timeout=self.smtpservertimeout)
             elif self.security == 'ssl':
+                if smtpcheckcert:
+                    msg = "Checking certificate is not supported for ssl, prefer starttls"
+                    raise smtplib.SMTPException(msg)
                 self.smtp = call(smtplib.SMTP_SSL, self.smtpserver, timeout=self.smtpservertimeout)
             elif self.security == 'tls':
                 if ':' not in self.smtpserver:
                     self.smtpserver += ':587'  # default port for TLS
                 self.smtp = call(smtplib.SMTP, self.smtpserver, timeout=self.smtpservertimeout)
-                self.smtp.ehlo()
-                self.smtp.starttls()
+                if smtpcheckcert:
+                    # inspired form:
+                    #   https://github.com/graingert/secure-smtplib/blob/master/src/secure_smtplib/__init__.py
+                    # but add the path to trusted ca, and force ceritficate verification.
+                    self.smtp.ehlo_or_helo_if_needed()
+                    if not self.smtp.has_extn("starttls"):
+                        msg = "STARTTLS extension not supported by server"
+                        raise smtplib.SMTPException(msg)
+                    (resp, reply) = self.smtp.docmd("STARTTLS")
+                    if resp == 220:
+                        self.smtp.sock = ssl.wrap_socket(
+                            self.smtp.sock,
+                            ca_certs=smtpcacerts,
+                            cert_reqs=ssl.CERT_REQUIRED
+                        )
+                        if not hasattr(self.smtp.sock, "read"):
+                            # using httplib.FakeSocket with Python 2.5.x or earlier
+                            self.smtp.sock.read = self.smtp.sock.recv
+                        self.smtp.file = smtplib.SSLFakeFile(self.smtp.sock)
+                        self.smtp.helo_resp = None
+                        self.smtp.ehlo_resp = None
+                        self.smtp.esmtp_features = {}
+                        self.smtp.does_esmtp = 0
+                    else:
+                        msg = "Wrong answer to the STARTTLS command"
+                        raise smtplib.SMTPException(msg)
+                else:
+                    self.smtp.ehlo()
+                    self.smtp.starttls()
                 self.smtp.ehlo()
             else:
                 sys.stdout.write('*** Error: Control reached an invalid option. ***')
@@ -3500,6 +3532,8 @@ def choose_mailer(config, environment):
         smtpencryption = config.get('smtpencryption', default='none')
         smtpuser = config.get('smtpuser', default='')
         smtppass = config.get('smtppass', default='')
+        smtpcacerts = config.get('smtpcacerts', default='/etc/ssl/certs/ca-certificates.crt')
+        smtpcheckcert = config.get_bool('smtpcheckcert', default='false')
         mailer = SMTPMailer(
             envelopesender=(environment.get_sender() or environment.get_fromaddr()),
             smtpserver=smtpserver, smtpservertimeout=smtpservertimeout,
@@ -3507,6 +3541,8 @@ def choose_mailer(config, environment):
             smtpencryption=smtpencryption,
             smtpuser=smtpuser,
             smtppass=smtppass,
+            smtpcacerts=smtpcacerts,
+            smtpcheckcert=smtpcheckcert
             )
     elif mailer == 'sendmail':
         command = config.get('sendmailcommand')