smtp: verify the certificate of the SMTP server for STARTTLS/SMTPS

Before this patch, the certificate of the SMTP server for STARTTLS or
SMTPS isn't verified.

This may cause man-in-the-middle security problem (stealing
authentication information), even though SMTP channel itself is
encrypted by SSL.

When "[smtp] tls" is configured as "smtps" or "starttls", this patch:

  - uses classes introduced by preceding patches instead of "SMTP" or
    "SMTP_SSL" of smtplib, and

  - verifies the certificate of the SMTP server, if "[smtp]
    verifycert" is configured as other than False

"[smtp] verifycert" can be configured in 3 levels:

  - "strict":

    This verifies peer certificate, and aborts if:

      - peer certification is not valid, or
      - no configuration in "[hostfingerprints]" and "[web] cacerts"

    This is default value of "[smtp] verifycert" for security.

  - "loose":

    This verifies peer certificate, and aborts if peer certification is
    not valid.

    This just shows warning message ("certificate not verified"), if
    there is no configuration in "[hostfingerprints]" and "[web]
    cacerts".

    This is as same as verification for HTTPS connection.

  - False(no verification):

    Peer certificate is not verified.

    This is as same as the behavior before this patch series.

"hg email --insecure" uses "loose" level, and ignores "[web] cacerts"
as same as push/pull/etc... with --insecure.

Ignoring "[web] cacerts" configuration for "hg email --insecure" is
already done in "dispatch._dispatch()" by looking "insecure" up in the
table of command options.

hgext/patchbomb.py

@@ -540,7 +540,13 b' def patchbomb(ui, repo, *revs, **opts):'
                 fp.close()
         else:
             if not sendmail:
-                sendmail = mail.connect(ui, mbox=mbox)
+                verifycert = ui.config('smtp', 'verifycert')
+                if opts.get('insecure'):
+                    ui.setconfig('smtp', 'verifycert', 'loose')
+                try:
+                    sendmail = mail.connect(ui, mbox=mbox)
+                finally:
+                    ui.setconfig('smtp', 'verifycert', verifycert)
             ui.status(_('sending '), subj, ' ...\n')
             ui.progress(_('sending'), i, item=subj, total=len(msgs))
             if not mbox:

mercurial/help/config.txt

@@ -1052,6 +1052,16 b' Configuration for extensions that need t'
     Optional. Method to enable TLS when connecting to mail server: starttls,
     smtps or none. Default: none.
 
+``verifycert``
+    Optional. Verification for the certificate of mail server, when
+    ``tls`` is starttls or smtps. "strict", "loose" or False. For
+    "strict" or "loose", the certificate is verified as same as the
+    verification for HTTPS connections (see ``[hostfingerprints]`` and
+    ``[web] cacerts`` also). For "strict", sending email is also
+    aborted, if there is no configuration for mail server in
+    ``[hostfingerprints]`` and ``[web] cacerts``.  --insecure for
+    :hg:`email` overwrites this as "loose". Default: "strict".
+
 ``username``
     Optional. User name for authenticating with the SMTP server.
     Default: none.

mercurial/mail.py

@@ -92,14 +92,25 b' def _smtp(ui):'
     smtps = tls == 'smtps'
     if (starttls or smtps) and not util.safehasattr(socket, 'ssl'):
         raise util.Abort(_("can't use TLS: Python SSL support not installed"))
-    if smtps:
-        ui.note(_('(using smtps)\n'))
-        s = smtplib.SMTP_SSL(local_hostname=local_hostname)
-    else:
-        s = smtplib.SMTP(local_hostname=local_hostname)
     mailhost = ui.config('smtp', 'host')
     if not mailhost:
         raise util.Abort(_('smtp.host not configured - cannot send mail'))
+    verifycert = ui.config('smtp', 'verifycert', 'strict')
+    if verifycert not in ['strict', 'loose']:
+        if util.parsebool(verifycert) is not False:
+            raise util.Abort(_('invalid smtp.verifycert configuration: %s')
+                             % (verifycert))
+    if (starttls or smtps) and verifycert:
+        sslkwargs = sslutil.sslkwargs(ui, mailhost)
+    else:
+        sslkwargs = {}
+    if smtps:
+        ui.note(_('(using smtps)\n'))
+        s = SMTPS(sslkwargs, local_hostname=local_hostname)
+    elif starttls:
+        s = STARTTLS(sslkwargs, local_hostname=local_hostname)
+    else:
+        s = smtplib.SMTP(local_hostname=local_hostname)
     mailport = util.getport(ui.config('smtp', 'port', 25))
     ui.note(_('sending mail: smtp host %s, port %s\n') %
             (mailhost, mailport))
@@ -109,6 +120,9 b' def _smtp(ui):'
         s.ehlo()
         s.starttls()
         s.ehlo()
+    if (starttls or smtps) and verifycert:
+        ui.note(_('(verifying remote certificate)\n'))
+        sslutil.validator(ui, mailhost)(s.sock, verifycert == 'strict')
     username = ui.config('smtp', 'username')
     password = ui.config('smtp', 'password')
     if username and not password: