bpo-43998: Default to TLS 1.2 and increase cipher suite security (GH-25778)

The ssl module now has more secure default settings. Ciphers without forward
secrecy or SHA-1 MAC are disabled by default. Security level 2 prohibits
weak RSA, DH, and ECC keys with less than 112 bits of security.
:class:`~ssl.SSLContext` defaults to minimum protocol version TLS 1.2.
Settings are based on Hynek Schlawack's research.

```
$ openssl version
OpenSSL 1.1.1k  FIPS 25 Mar 2021
$ openssl ciphers -v '@SECLEVEL=2:ECDH+AESGCM:ECDH+CHACHA20:ECDH+AES:DHE+AES:!aNULL:!eNULL:!aDSS:!SHA1:!AESCCM'
TLS_AES_256_GCM_SHA384  TLSv1.3 Kx=any      Au=any  Enc=AESGCM(256) Mac=AEAD
TLS_CHACHA20_POLY1305_SHA256 TLSv1.3 Kx=any      Au=any  Enc=CHACHA20/POLY1305(256) Mac=AEAD
TLS_AES_128_GCM_SHA256  TLSv1.3 Kx=any      Au=any  Enc=AESGCM(128) Mac=AEAD
TLS_AES_128_CCM_SHA256  TLSv1.3 Kx=any      Au=any  Enc=AESCCM(128) Mac=AEAD
ECDHE-ECDSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AESGCM(256) Mac=AEAD
ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AESGCM(256) Mac=AEAD
ECDHE-ECDSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AESGCM(128) Mac=AEAD
ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AESGCM(128) Mac=AEAD
ECDHE-ECDSA-CHACHA20-POLY1305 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=CHACHA20/POLY1305(256) Mac=AEAD
ECDHE-RSA-CHACHA20-POLY1305 TLSv1.2 Kx=ECDH     Au=RSA  Enc=CHACHA20/POLY1305(256) Mac=AEAD
ECDHE-ECDSA-AES256-SHA384 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AES(256)  Mac=SHA384
ECDHE-RSA-AES256-SHA384 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AES(256)  Mac=SHA384
ECDHE-ECDSA-AES128-SHA256 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AES(128)  Mac=SHA256
ECDHE-RSA-AES128-SHA256 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AES(128)  Mac=SHA256
DHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=DH       Au=RSA  Enc=AESGCM(256) Mac=AEAD
DHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=DH       Au=RSA  Enc=AESGCM(128) Mac=AEAD
DHE-RSA-AES256-SHA256   TLSv1.2 Kx=DH       Au=RSA  Enc=AES(256)  Mac=SHA256
DHE-RSA-AES128-SHA256   TLSv1.2 Kx=DH       Au=RSA  Enc=AES(128)  Mac=SHA256
```

Signed-off-by: Christian Heimes <christian@python.org>

6 files changed, 76 insertions, 17 deletions

diff --git a/Doc/library/ssl.rst b/Doc/library/ssl.rst
index af9bf798da3..0166fe3caca 100644
--- a/Doc/library/ssl.rst
+++ b/Doc/library/ssl.rst
@@ -1471,6 +1471,14 @@ to speed up repeated connections from the same clients.
       ciphers, no ``NULL`` ciphers and no ``MD5`` ciphers (except for
       :data:`PROTOCOL_SSLv2`).
 
+   .. versionchanged:: 3.9.5_p2 (Gentoo)
+
+      The default cipher suites now include only secure AES and ChaCha20
+      ciphers with forward secrecy and security level 2. RSA and DH keys with
+      less than 2048 bits and ECC keys with less than 224 bits are prohibited.
+      :data:`PROTOCOL_TLS`, :data:`PROTOCOL_TLS_CLIENT`, and
+      :data:`PROTOCOL_TLS_SERVER` use TLS 1.2 as minimum TLS version.
+
 
 :class:`SSLContext` objects have the following methods and attributes:
 
diff --git a/Lib/test/test_nntplib.py b/Lib/test/test_nntplib.py
index 99386ddbaed..cc7c93821d7 100644
--- a/Lib/test/test_nntplib.py
+++ b/Lib/test/test_nntplib.py
@@ -37,6 +37,8 @@ else:
 
 class NetworkedNNTPTestsMixin:
 
+    ssl_context = None
+
     def test_welcome(self):
         welcome = self.server.getwelcome()
         self.assertEqual(str, type(welcome))
@@ -273,18 +275,21 @@ class NetworkedNNTPTestsMixin:
                 return False
             return True
 
+        kwargs = dict(
+            timeout=support.INTERNET_TIMEOUT,
+            usenetrc=False
+        )
+        if self.ssl_context is not None:
+            kwargs["ssl_context"] = self.ssl_context
+
         try:
-            server = self.NNTP_CLASS(self.NNTP_HOST,
-                                     timeout=support.INTERNET_TIMEOUT,
-                                     usenetrc=False)
+            server = self.NNTP_CLASS(self.NNTP_HOST, **kwargs)
             with server:
                 self.assertTrue(is_connected())
                 self.assertTrue(server.help())
             self.assertFalse(is_connected())
 
-            server = self.NNTP_CLASS(self.NNTP_HOST,
-                                     timeout=support.INTERNET_TIMEOUT,
-                                     usenetrc=False)
+            server = self.NNTP_CLASS(self.NNTP_HOST, **kwargs)
             with server:
                 server.quit()
             self.assertFalse(is_connected())
@@ -316,16 +321,21 @@ class NetworkedNNTPTests(NetworkedNNTPTestsMixin, unittest.TestCase):
     @classmethod
     def setUpClass(cls):
         support.requires("network")
+        kwargs = dict(
+            timeout=support.INTERNET_TIMEOUT,
+            usenetrc=False
+        )
+        if cls.ssl_context is not None:
+            kwargs["ssl_context"] = cls.ssl_context
         with socket_helper.transient_internet(cls.NNTP_HOST):
             try:
-                cls.server = cls.NNTP_CLASS(cls.NNTP_HOST,
-                                            timeout=support.INTERNET_TIMEOUT,
-                                            usenetrc=False)
+                cls.server = cls.NNTP_CLASS(cls.NNTP_HOST, **kwargs)
             except SSLError as ssl_err:
                 # matches "[SSL: DH_KEY_TOO_SMALL] dh key too small"
                 if re.search(r'(?i)KEY.TOO.SMALL', ssl_err.reason):
                     raise unittest.SkipTest(f"{cls} got {ssl_err} connecting "
                                             f"to {cls.NNTP_HOST!r}")
+                print(cls.NNTP_HOST)
                 raise
             except EOF_ERRORS:
                 raise unittest.SkipTest(f"{cls} got EOF error on connecting "
@@ -358,6 +368,9 @@ class NetworkedNNTP_SSLTests(NetworkedNNTPTests):
     # Disabled as the connection will already be encrypted.
     test_starttls = None
 
+    ssl_context = ssl._create_unverified_context()
+    ssl_context.set_ciphers("DEFAULT")
+    ssl_context.maximum_version = ssl.TLSVersion.TLSv1_2
 
 #
 # Non-networked tests using a local server (or something mocking it).
diff --git a/Misc/NEWS.d/next/Security/2021-05-01-13-13-40.bpo-43998.xhmWD7.rst b/Misc/NEWS.d/next/Security/2021-05-01-13-13-40.bpo-43998.xhmWD7.rst
new file mode 100644
index 00000000000..6a40346128e
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2021-05-01-13-13-40.bpo-43998.xhmWD7.rst
@@ -0,0 +1,5 @@
+The :mod:`ssl` module sets more secure cipher suites defaults. Ciphers
+without forward secrecy and with SHA-1 MAC are disabled by default. Security
+level 2 prohibits weak RSA, DH, and ECC keys with less than 112 bits of
+security. :class:`~ssl.SSLContext` defaults to minimum protocol version TLS
+1.2. Settings are based on Hynek Schlawack's research.
diff --git a/Modules/_ssl.c b/Modules/_ssl.c
index 52bd6841faf..c39d6ba4e95 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -298,15 +298,27 @@ SSL_SESSION_get_ticket_lifetime_hint(const SSL_SESSION *s)
   #ifndef PY_SSL_DEFAULT_CIPHER_STRING
      #error "Py_SSL_DEFAULT_CIPHERS 0 needs Py_SSL_DEFAULT_CIPHER_STRING"
   #endif
+  #ifndef PY_SSL_MIN_PROTOCOL
+    #define PY_SSL_MIN_PROTOCOL TLS1_2_VERSION
+  #endif
 #elif PY_SSL_DEFAULT_CIPHERS == 1
 /* Python custom selection of sensible cipher suites
- * DEFAULT: OpenSSL's default cipher list. Since 1.0.2 the list is in sensible order.
+ * @SECLEVEL=2: security level 2 with 112 bits minimum security (e.g. 2048 bits RSA key)
+ * ECDH+*: enable ephemeral elliptic curve Diffie-Hellman
+ * DHE+*: fallback to ephemeral finite field Diffie-Hellman
+ * encryption order: AES AEAD (GCM), ChaCha AEAD, AES CBC
  * !aNULL:!eNULL: really no NULL ciphers
- * !MD5:!3DES:!DES:!RC4:!IDEA:!SEED: no weak or broken algorithms on old OpenSSL versions.
  * !aDSS: no authentication with discrete logarithm DSA algorithm
- * !SRP:!PSK: no secure remote password or pre-shared key authentication
+ * !SHA1: no weak SHA1 MAC
+ * !AESCCM: no CCM mode, it's uncommon and slow
+ *
+ * Based on Hynek's excellent blog post (update 2021-02-11)
+ * https://hynek.me/articles/hardening-your-web-servers-ssl-ciphers/
  */
-  #define PY_SSL_DEFAULT_CIPHER_STRING "DEFAULT:!aNULL:!eNULL:!MD5:!3DES:!DES:!RC4:!IDEA:!SEED:!aDSS:!SRP:!PSK"
+  #define PY_SSL_DEFAULT_CIPHER_STRING "@SECLEVEL=2:ECDH+AESGCM:ECDH+CHACHA20:ECDH+AES:DHE+AES:!aNULL:!eNULL:!aDSS:!SHA1:!AESCCM"
+  #ifndef PY_SSL_MIN_PROTOCOL
+    #define PY_SSL_MIN_PROTOCOL TLS1_2_VERSION
+  #endif
 #elif PY_SSL_DEFAULT_CIPHERS == 2
 /* Ignored in SSLContext constructor, only used to as _ssl.DEFAULT_CIPHER_STRING */
   #define PY_SSL_DEFAULT_CIPHER_STRING SSL_DEFAULT_CIPHER_LIST
@@ -3251,8 +3263,25 @@ _ssl__SSLContext_impl(PyTypeObject *type, int proto_version)
         ERR_clear_error();
         PyErr_SetString(PySSLErrorObject,
                         "No cipher can be selected.");
-        return NULL;
+        goto error;
+    }
+#ifdef PY_SSL_MIN_PROTOCOL
+    switch(proto_version) {
+    case PY_SSL_VERSION_TLS:
+    case PY_SSL_VERSION_TLS_CLIENT:
+    case PY_SSL_VERSION_TLS_SERVER:
+        result = SSL_CTX_set_min_proto_version(ctx, PY_SSL_MIN_PROTOCOL);
+        if (result == 0) {
+            PyErr_Format(PyExc_ValueError,
+                         "Failed to set minimum protocol 0x%x",
+                          PY_SSL_MIN_PROTOCOL);
+            goto error;
+        }
+        break;
+    default:
+        break;
     }
+#endif
 
 #if defined(SSL_MODE_RELEASE_BUFFERS)
     /* Set SSL_MODE_RELEASE_BUFFERS. This potentially greatly reduces memory
@@ -3305,6 +3334,10 @@ _ssl__SSLContext_impl(PyTypeObject *type, int proto_version)
 #endif
 
     return (PyObject *)self;
+  error:
+    Py_XDECREF(self);
+    ERR_clear_error();
+    return NULL;
 }
 
 static int
diff --git a/configure b/configure
index b7be60eaa37..a6c70d3ee16 100755
--- a/configure
+++ b/configure
@@ -1596,8 +1596,8 @@ Optional Packages:
                           override default cipher suites string, python: use
                           Python's preferred selection (default), openssl:
                           leave OpenSSL's defaults untouched, STRING: use a
-                          custom string, PROTOCOL_SSLv2 ignores the setting,
-                          see Doc/library/ssl.rst
+                          custom string, python and STRING also set TLS 1.2 as
+                          minimum TLS version
   --with-builtin-hashlib-hashes=md5,sha1,sha256,sha512,sha3,blake2
                           builtin hash modules, md5, sha1, sha256, sha512,
                           sha3 (with shake), blake2
diff --git a/configure.ac b/configure.ac
index aa515da4655..a760b508986 100644
--- a/configure.ac
+++ b/configure.ac
@@ -5807,7 +5807,7 @@ AC_ARG_WITH(ssl-default-suites,
                             python: use Python's preferred selection (default),
                             openssl: leave OpenSSL's defaults untouched,
                             STRING: use a custom string,
-                            PROTOCOL_SSLv2 ignores the setting, see Doc/library/ssl.rst]),
+                            python and STRING also set TLS 1.2 as minimum TLS version]),
 [
 AC_MSG_RESULT($withval)
 case "$withval" in