[2.7] bpo-28043: improved default settings for SSLContext (GH-10608)

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).

(cherry picked from commit 358cfd426ccc0fcd6a7940d306602138e76420ae)

1 files changed, 6 insertions, 24 deletions

diff --git a/Lib/ssl.py b/Lib/ssl.py
index 087faf9..0bb43a4 100644
--- a/Lib/ssl.py
+++ b/Lib/ssl.py
@@ -424,32 +424,16 @@ 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:
@@ -475,12 +459,10 @@ 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