[bpo-28414] In SSL module, store server_hostname as an A-label

Doc/library/ssl.rst

@@ -1247,6 +1247,12 @@ SSL sockets also have the following additional methods and attributes:
 
    .. versionadded:: 3.2
 
+   .. versionchanged:: 3.7
+      When ``server_hostname`` is an internationalized domain name
+      (IDN), this attribute now stores the A-label form
+      (``"xn--pythn-mua.org"``), rather than the U-label form
+      (``"pythön.org"``).
+
 .. attribute:: SSLSocket.session
 
    The :class:`SSLSession` for this SSL connection. The session is available

Doc/whatsnew/3.7.rst

@@ -430,6 +430,16 @@ can be set within the scope of a group.
 ``'^$'`` or ``(?=-)`` that matches an empty string.
 (Contributed by Serhiy Storchaka in :issue:`25054`.)
 
+ssl
+---
+
+Added support for validating server certificates containing
+internationalized domain names (IDNs). As part of this change, the
+:attr:`ssl.SSLSocket.server_hostname` attribute now stores the
+expected hostname in A-label form (``"xn--pythn-mua.org"``), rather
+than the U-label form (``"pythön.org"``). (Contributed by
+Nathaniel J. Smith in :issue:`28414`.)
+
 string
 ------
 

Lib/test/ssl-idn-ca.pem

@@ -0,0 +1,15 @@
+-----BEGIN CERTIFICATE-----
+MIICQDCCAamgAwIBAgIUBg5Z+vupJbrxjKmrYeJ6nb3xnQEwDQYJKoZIhvcNAQEL
+BQAwQDEXMBUGA1UECgwOdHJ1c3RtZSB2MC40LjAxJTAjBgNVBAsMHFRlc3Rpbmcg
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+dcUonA==
+-----END CERTIFICATE-----

Lib/test/ssl-idn-cert.pem

@@ -0,0 +1,31 @@
+-----BEGIN RSA PRIVATE KEY-----
+MIICXQIBAAKBgQCjbzNwZrxp50RAVF55jSw5M//KD5/kswwdpUePux00JS5WnHh3
+SE98rnrWS34ryblBEMOEgIZPuYFjLs0fVN1XgmUHxs2cDPFUOpBrK2tf+nMDN0o9
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+rKdC2AdCvu3FZCbNg0lpjiYXEYCXvCP4c5TqS3H8uaC8
+-----END RSA PRIVATE KEY-----
+-----BEGIN CERTIFICATE-----
+MIICjTCCAfagAwIBAgIUHg5Af4BmKSy4Xyot/yAGeSkO+PswDQYJKoZIhvcNAQEL
+BQAwQDEXMBUGA1UECgwOdHJ1c3RtZSB2MC40LjAxJTAjBgNVBAsMHFRlc3Rpbmcg
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+vpg6hKEGXN0UjYPb5HBPFKsWF0DbbNaWrr0co32yH2L8
+-----END CERTIFICATE-----

Lib/test/test_ssl.py

@@ -80,6 +80,15 @@ def data_file(*name):
 DHFILE = data_file("dh1024.pem")
 BYTES_DHFILE = os.fsencode(DHFILE)
 
+# These were generated by doing 'pip install trustme', and then:
+# import trustme
+# ca = trustme.CA()
+# cert = ca.issue_server_cert("pythön.example.org")
+# ca.cert_pem.write_to_path("ssl-idn-ca.pem")
+# cert.private_key_and_cert_chain_pem.write_to_path("ssl-idn-cert.pem")
+IDN_CA = data_file("ssl-idn-ca.pem")
+IDN_CERT = data_file("ssl-idn-cert.pem")
+
 # Not defined in all versions of OpenSSL
 OP_NO_COMPRESSION = getattr(ssl, "OP_NO_COMPRESSION", 0)
 OP_SINGLE_DH_USE = getattr(ssl, "OP_SINGLE_DH_USE", 0)
@@ -1474,16 +1483,6 @@ def test_subclass(self):
                 # For compatibility
                 self.assertEqual(cm.exception.errno, ssl.SSL_ERROR_WANT_READ)
 
-    def test_bad_idna_in_server_hostname(self):
-        # Note: this test is testing some code that probably shouldn't exist
-        # in the first place, so if it starts failing at some point because
-        # you made the ssl module stop doing IDNA decoding then please feel
-        # free to remove it. The test was mainly added because this case used
-        # to cause memory corruption (see bpo-30594).
-        ctx = ssl.create_default_context()
-        with self.assertRaises(UnicodeError):
-            ctx.wrap_bio(ssl.MemoryBIO(), ssl.MemoryBIO(),
-                         server_hostname="xn--.com")
 
 class MemoryBIOTests(unittest.TestCase):
 
@@ -2522,6 +2521,39 @@ def test_check_hostname(self):
                                             "check_hostname requires server_hostname"):
                     client_context.wrap_socket(s)
 
+    def test_check_hostname_idn(self):
+        if support.verbose:
+            sys.stdout.write("\n")
+
+        server_context = ssl.SSLContext(ssl.PROTOCOL_TLS)
+        server_context.load_cert_chain(IDN_CERT)
+
+        context = ssl.SSLContext(ssl.PROTOCOL_TLS)
+        context.verify_mode = ssl.CERT_REQUIRED
+        context.check_hostname = True
+        context.load_verify_locations(IDN_CA)
+
+        # correct hostname should verify, when specified in several
+        # different ways
+        for server_hostname in ["pythön.example.org",
+                                "xn--pythn-mua.example.org",
+                                b"xn--pythn-mua.example.org"]:
+            server = ThreadedEchoServer(context=server_context, chatty=True)
+            with server:
+                with context.wrap_socket(socket.socket(),
+                                         server_hostname=server_hostname) as s:
+                    s.connect((HOST, server.port))
+                    cert = s.getpeercert()
+                    self.assertTrue(cert, "Can't get peer certificate.")
+
+        # incorrect hostname should raise an exception
+        server = ThreadedEchoServer(context=server_context, chatty=True)
+        with server:
+            with context.wrap_socket(socket.socket(),
+                                     server_hostname="python.example.org") as s:
+                with self.assertRaises(ssl.CertificateError):
+                    s.connect((HOST, server.port))
+
     def test_wrong_cert(self):
         """Connecting when the server rejects the client's certificate
 

Misc/NEWS.d/next/Security/2017-08-06-14-43-45.bpo-28414.mzZ6vD.rst

@@ -0,0 +1,2 @@
+The ssl module can now validate hostnames that contain non-ASCII
+characters (IDNs).

Modules/_ssl.c

@@ -716,8 +716,11 @@ newPySSLSocket(PySSLContext *sslctx, PySocketSockObject *sock,
     self->owner = NULL;
     self->server_hostname = NULL;
     if (server_hostname != NULL) {
+        /* server_hostname was encoded to an A-label by our caller; put it
+         * back into a str object, but still as an A-label (bpo-28414)
+         */
         PyObject *hostname = PyUnicode_Decode(server_hostname, strlen(server_hostname),
-                                              "idna", "strict");
+                                              "ascii", "strict");
         if (hostname == NULL) {
             Py_DECREF(self);
             return NULL;