Issue 28043: SSLContext has improved default settings

The options OP_NO_COMPRESSION, OP_CIPHER_SERVER_PREFERENCE, OP_SINGLE_DH_USE, OP_SINGLE_ECDH_USE, OP_NO_SSLv2 (except for PROTOCOL_SSLv2), and OP_NO_SSLv3 (except for PROTOCOL_SSLv3) are set by default. The initial cipher suite list contains only HIGH ciphers, no NULL ciphers and MD5 ciphers (except for PROTOCOL_SSLv2).

Doc/library/ssl.rst

@@ -1191,7 +1191,14 @@ to speed up repeated connections from the same clients.
 
    .. versionchanged:: 3.6
 
-      :data:`PROTOCOL_TLS` is the default value.
+      The context is created with secure default values. The options
+      :data:`OP_NO_COMPRESSION`, :data:`OP_CIPHER_SERVER_PREFERENCE`,
+      :data:`OP_SINGLE_DH_USE`, :data:`OP_SINGLE_ECDH_USE`,
+      :data:`OP_NO_SSLv2` (except for :data:`PROTOCOL_SSLv2`),
+      and :data:`OP_NO_SSLv3` (except for :data:`PROTOCOL_SSLv3`) are
+      set by default. The initial cipher suite list contains only ``HIGH``
+      ciphers, no ``NULL`` ciphers and no ``MD5`` ciphers (except for
+      :data:`PROTOCOL_SSLv2`).
 
 
 :class:`SSLContext` objects have the following methods and attributes:

Lib/ssl.py

@@ -488,32 +488,16 @@ def create_default_context(purpose=Purpose.SERVER_AUTH, *, cafile=None,
     if not isinstance(purpose, _ASN1Object):
         raise TypeError(purpose)
 
+    # SSLContext sets OP_NO_SSLv2, OP_NO_SSLv3, OP_NO_COMPRESSION,
+    # OP_CIPHER_SERVER_PREFERENCE, OP_SINGLE_DH_USE and OP_SINGLE_ECDH_USE
+    # by default.
     context = SSLContext(PROTOCOL_TLS)
 
-    # SSLv2 considered harmful.
-    context.options |= OP_NO_SSLv2
-
-    # SSLv3 has problematic security and is only required for really old
-    # clients such as IE6 on Windows XP
-    context.options |= OP_NO_SSLv3
-
-    # disable compression to prevent CRIME attacks (OpenSSL 1.0+)
-    context.options |= getattr(_ssl, "OP_NO_COMPRESSION", 0)
-
     if purpose == Purpose.SERVER_AUTH:
         # verify certs and host name in client mode
         context.verify_mode = CERT_REQUIRED
         context.check_hostname = True
     elif purpose == Purpose.CLIENT_AUTH:
-        # Prefer the server's ciphers by default so that we get stronger
-        # encryption
-        context.options |= getattr(_ssl, "OP_CIPHER_SERVER_PREFERENCE", 0)
-
-        # Use single use keys in order to improve forward secrecy
-        context.options |= getattr(_ssl, "OP_SINGLE_DH_USE", 0)
-        context.options |= getattr(_ssl, "OP_SINGLE_ECDH_USE", 0)
-
-        # disallow ciphers with known vulnerabilities
         context.set_ciphers(_RESTRICTED_SERVER_CIPHERS)
 
     if cafile or capath or cadata:
@@ -539,12 +523,10 @@ def _create_unverified_context(protocol=PROTOCOL_TLS, *, cert_reqs=None,
     if not isinstance(purpose, _ASN1Object):
         raise TypeError(purpose)
 
+    # SSLContext sets OP_NO_SSLv2, OP_NO_SSLv3, OP_NO_COMPRESSION,
+    # OP_CIPHER_SERVER_PREFERENCE, OP_SINGLE_DH_USE and OP_SINGLE_ECDH_USE
+    # by default.
     context = SSLContext(protocol)
-    # SSLv2 considered harmful.
-    context.options |= OP_NO_SSLv2
-    # SSLv3 has problematic security and is only required for really old
-    # clients such as IE6 on Windows XP
-    context.options |= OP_NO_SSLv3
 
     if cert_reqs is not None:
         context.verify_mode = cert_reqs

Lib/test/test_ssl.py

@@ -80,6 +80,12 @@ NULLBYTECERT = data_file("nullbytecert.pem")
 DHFILE = data_file("dh1024.pem")
 BYTES_DHFILE = os.fsencode(DHFILE)
 
+# 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)
+OP_SINGLE_ECDH_USE = getattr(ssl, "OP_SINGLE_ECDH_USE", 0)
+OP_CIPHER_SERVER_PREFERENCE = getattr(ssl, "OP_CIPHER_SERVER_PREFERENCE", 0)
+
 
 def handle_error(prefix):
     exc_format = ' '.join(traceback.format_exception(*sys.exc_info()))
@@ -870,8 +876,9 @@ class ContextTests(unittest.TestCase):
         ctx = ssl.SSLContext(ssl.PROTOCOL_TLSv1)
         # OP_ALL | OP_NO_SSLv2 | OP_NO_SSLv3 is the default value
         default = (ssl.OP_ALL | ssl.OP_NO_SSLv2 | ssl.OP_NO_SSLv3)
-        if not IS_LIBRESSL and ssl.OPENSSL_VERSION_INFO >= (1, 1, 0):
-            default |= ssl.OP_NO_COMPRESSION
+        # SSLContext also enables these by default
+        default |= (OP_NO_COMPRESSION | OP_CIPHER_SERVER_PREFERENCE |
+                    OP_SINGLE_DH_USE | OP_SINGLE_ECDH_USE)
         self.assertEqual(default, ctx.options)
         ctx.options |= ssl.OP_NO_TLSv1
         self.assertEqual(default | ssl.OP_NO_TLSv1, ctx.options)
@@ -1236,16 +1243,29 @@ class ContextTests(unittest.TestCase):
             stats["x509"] += 1
             self.assertEqual(ctx.cert_store_stats(), stats)
 
+    def _assert_context_options(self, ctx):
+        self.assertEqual(ctx.options & ssl.OP_NO_SSLv2, ssl.OP_NO_SSLv2)
+        if OP_NO_COMPRESSION != 0:
+            self.assertEqual(ctx.options & OP_NO_COMPRESSION,
+                             OP_NO_COMPRESSION)
+        if OP_SINGLE_DH_USE != 0:
+            self.assertEqual(ctx.options & OP_SINGLE_DH_USE,
+                             OP_SINGLE_DH_USE)
+        if OP_SINGLE_ECDH_USE != 0:
+            self.assertEqual(ctx.options & OP_SINGLE_ECDH_USE,
+                             OP_SINGLE_ECDH_USE)
+        if OP_CIPHER_SERVER_PREFERENCE != 0:
+            self.assertEqual(ctx.options & OP_CIPHER_SERVER_PREFERENCE,
+                             OP_CIPHER_SERVER_PREFERENCE)
+
     def test_create_default_context(self):
         ctx = ssl.create_default_context()
+
         self.assertEqual(ctx.protocol, ssl.PROTOCOL_SSLv23)
         self.assertEqual(ctx.verify_mode, ssl.CERT_REQUIRED)
         self.assertTrue(ctx.check_hostname)
-        self.assertEqual(ctx.options & ssl.OP_NO_SSLv2, ssl.OP_NO_SSLv2)
-        self.assertEqual(
-            ctx.options & getattr(ssl, "OP_NO_COMPRESSION", 0),
-            getattr(ssl, "OP_NO_COMPRESSION", 0),
-        )
+        self._assert_context_options(ctx)
+
 
         with open(SIGNING_CA) as f:
             cadata = f.read()
@@ -1253,40 +1273,24 @@ class ContextTests(unittest.TestCase):
                                          cadata=cadata)
         self.assertEqual(ctx.protocol, ssl.PROTOCOL_SSLv23)
         self.assertEqual(ctx.verify_mode, ssl.CERT_REQUIRED)
-        self.assertEqual(ctx.options & ssl.OP_NO_SSLv2, ssl.OP_NO_SSLv2)
-        self.assertEqual(
-            ctx.options & getattr(ssl, "OP_NO_COMPRESSION", 0),
-            getattr(ssl, "OP_NO_COMPRESSION", 0),
-        )
+        self._assert_context_options(ctx)
 
         ctx = ssl.create_default_context(ssl.Purpose.CLIENT_AUTH)
         self.assertEqual(ctx.protocol, ssl.PROTOCOL_SSLv23)
         self.assertEqual(ctx.verify_mode, ssl.CERT_NONE)
-        self.assertEqual(ctx.options & ssl.OP_NO_SSLv2, ssl.OP_NO_SSLv2)
-        self.assertEqual(
-            ctx.options & getattr(ssl, "OP_NO_COMPRESSION", 0),
-            getattr(ssl, "OP_NO_COMPRESSION", 0),
-        )
-        self.assertEqual(
-            ctx.options & getattr(ssl, "OP_SINGLE_DH_USE", 0),
-            getattr(ssl, "OP_SINGLE_DH_USE", 0),
-        )
-        self.assertEqual(
-            ctx.options & getattr(ssl, "OP_SINGLE_ECDH_USE", 0),
-            getattr(ssl, "OP_SINGLE_ECDH_USE", 0),
-        )
+        self._assert_context_options(ctx)
 
     def test__create_stdlib_context(self):
         ctx = ssl._create_stdlib_context()
         self.assertEqual(ctx.protocol, ssl.PROTOCOL_SSLv23)
         self.assertEqual(ctx.verify_mode, ssl.CERT_NONE)
         self.assertFalse(ctx.check_hostname)
-        self.assertEqual(ctx.options & ssl.OP_NO_SSLv2, ssl.OP_NO_SSLv2)
+        self._assert_context_options(ctx)
 
         ctx = ssl._create_stdlib_context(ssl.PROTOCOL_TLSv1)
         self.assertEqual(ctx.protocol, ssl.PROTOCOL_TLSv1)
         self.assertEqual(ctx.verify_mode, ssl.CERT_NONE)
-        self.assertEqual(ctx.options & ssl.OP_NO_SSLv2, ssl.OP_NO_SSLv2)
+        self._assert_context_options(ctx)
 
         ctx = ssl._create_stdlib_context(ssl.PROTOCOL_TLSv1,
                                          cert_reqs=ssl.CERT_REQUIRED,
@@ -1294,12 +1298,12 @@ class ContextTests(unittest.TestCase):
         self.assertEqual(ctx.protocol, ssl.PROTOCOL_TLSv1)
         self.assertEqual(ctx.verify_mode, ssl.CERT_REQUIRED)
         self.assertTrue(ctx.check_hostname)
-        self.assertEqual(ctx.options & ssl.OP_NO_SSLv2, ssl.OP_NO_SSLv2)
+        self._assert_context_options(ctx)
 
         ctx = ssl._create_stdlib_context(purpose=ssl.Purpose.CLIENT_AUTH)
         self.assertEqual(ctx.protocol, ssl.PROTOCOL_SSLv23)
         self.assertEqual(ctx.verify_mode, ssl.CERT_NONE)
-        self.assertEqual(ctx.options & ssl.OP_NO_SSLv2, ssl.OP_NO_SSLv2)
+        self._assert_context_options(ctx)
 
     def test_check_hostname(self):
         ctx = ssl.SSLContext(ssl.PROTOCOL_TLSv1)

Misc/NEWS

@@ -138,6 +138,10 @@ Core and Builtins
 Library
 -------
 
+- Issue 28043: SSLContext has improved default settings: OP_NO_SSLv2,
+  OP_NO_SSLv3, OP_NO_COMPRESSION, OP_CIPHER_SERVER_PREFERENCE,
+  OP_SINGLE_DH_USE, OP_SINGLE_ECDH_USE and HIGH ciphers without MD5.
+
 - Issue #24693: Changed some RuntimeError's in the zipfile module to more
   appropriate types. Improved some error messages and debugging output.
 

Modules/_ssl.c

@@ -2407,6 +2407,7 @@ _ssl__SSLContext_impl(PyTypeObject *type, int proto_version)
     PySSLContext *self;
     long options;
     SSL_CTX *ctx = NULL;
+    int result;
 #if defined(SSL_MODE_RELEASE_BUFFERS)
     unsigned long libver;
 #endif
@@ -2470,8 +2471,38 @@ _ssl__SSLContext_impl(PyTypeObject *type, int proto_version)
         options |= SSL_OP_NO_SSLv2;
     if (proto_version != PY_SSL_VERSION_SSL3)
         options |= SSL_OP_NO_SSLv3;
+    /* Minimal security flags for server and client side context.
+     * Client sockets ignore server-side parameters. */
+#ifdef SSL_OP_NO_COMPRESSION
+    options |= SSL_OP_NO_COMPRESSION;
+#endif
+#ifdef SSL_OP_CIPHER_SERVER_PREFERENCE
+    options |= SSL_OP_CIPHER_SERVER_PREFERENCE;
+#endif
+#ifdef SSL_OP_SINGLE_DH_USE
+    options |= SSL_OP_SINGLE_DH_USE;
+#endif
+#ifdef SSL_OP_SINGLE_ECDH_USE
+    options |= SSL_OP_SINGLE_ECDH_USE;
+#endif
     SSL_CTX_set_options(self->ctx, options);
 
+    /* A bare minimum cipher list without completly broken cipher suites.
+     * It's far from perfect but gives users a better head start. */
+    if (proto_version != PY_SSL_VERSION_SSL2) {
+        result = SSL_CTX_set_cipher_list(ctx, "HIGH:!aNULL:!eNULL:!MD5");
+    } else {
+        /* SSLv2 needs MD5 */
+        result = SSL_CTX_set_cipher_list(ctx, "HIGH:!aNULL:!eNULL");
+    }
+    if (result == 0) {
+        Py_DECREF(self);
+        ERR_clear_error();
+        PyErr_SetString(PySSLErrorObject,
+                        "No cipher can be selected.");
+        return NULL;
+    }
+
 #if defined(SSL_MODE_RELEASE_BUFFERS)
     /* Set SSL_MODE_RELEASE_BUFFERS. This potentially greatly reduces memory
        usage for no cost at all. However, don't do this for OpenSSL versions