[Python-checkins] bpo-31429: Define TLS cipher suite on build time (#3532)
Christian Heimes
webhook-mailer at python.org
Mon Jan 29 08:10:21 EST 2018
https://github.com/python/cpython/commit/892d66e422d5367673163d62ba40cd70a37d5cf7
commit: 892d66e422d5367673163d62ba40cd70a37d5cf7
branch: master
author: Christian Heimes <christian at python.org>
committer: GitHub <noreply at github.com>
date: 2018-01-29T14:10:18+01:00
summary:
bpo-31429: Define TLS cipher suite on build time (#3532)
Until now Python used a hard coded white list of default TLS cipher
suites. The old approach has multiple downsides. OpenSSL's default
selection was completely overruled. Python did neither benefit from new
cipher suites (ChaCha20, TLS 1.3 suites) nor blacklisted cipher suites.
For example we used to re-enable 3DES.
Python now defaults to OpenSSL DEFAULT cipher suite selection and black
lists all unwanted ciphers. Downstream vendors can override the default
cipher list with --with-ssl-default-suites.
Signed-off-by: Christian Heimes <christian at python.org>
files:
A Misc/NEWS.d/next/Library/2018-01-28-22-40-05.bpo-31429.qNt8rQ.rst
M Doc/whatsnew/3.7.rst
M Lib/ssl.py
M Lib/test/test_ssl.py
M Modules/_ssl.c
M configure
M configure.ac
M pyconfig.h.in
diff --git a/Doc/whatsnew/3.7.rst b/Doc/whatsnew/3.7.rst
index b70945f4708e..e36e505fd8ae 100644
--- a/Doc/whatsnew/3.7.rst
+++ b/Doc/whatsnew/3.7.rst
@@ -623,6 +623,12 @@ wildcard matching disabled by default.
(Contributed by Mandeep Singh in :issue:`23033` and Christian Heimes in
:issue:`31399`.)
+The default cipher suite selection of the ssl module now uses a blacklist
+approach rather than a hard-coded whitelist. Python no longer re-enables
+ciphers that have been blocked by OpenSSL security update. Default cipher
+suite selection can be configured on compile time.
+(Contributed by Christian Heimes in :issue:`31429`.)
+
string
------
diff --git a/Lib/ssl.py b/Lib/ssl.py
index 5f972e1d3d8a..b6161d0f178d 100644
--- a/Lib/ssl.py
+++ b/Lib/ssl.py
@@ -115,6 +115,7 @@
from _ssl import HAS_SNI, HAS_ECDH, HAS_NPN, HAS_ALPN, HAS_TLSv1_3
+from _ssl import _DEFAULT_CIPHERS
from _ssl import _OPENSSL_API_VERSION
@@ -174,48 +175,7 @@
HAS_NEVER_CHECK_COMMON_NAME = hasattr(_ssl, 'HOSTFLAG_NEVER_CHECK_SUBJECT')
-# Disable weak or insecure ciphers by default
-# (OpenSSL's default setting is 'DEFAULT:!aNULL:!eNULL')
-# Enable a better set of ciphers by default
-# This list has been explicitly chosen to:
-# * TLS 1.3 ChaCha20 and AES-GCM cipher suites
-# * Prefer cipher suites that offer perfect forward secrecy (DHE/ECDHE)
-# * Prefer ECDHE over DHE for better performance
-# * Prefer AEAD over CBC for better performance and security
-# * Prefer AES-GCM over ChaCha20 because most platforms have AES-NI
-# (ChaCha20 needs OpenSSL 1.1.0 or patched 1.0.2)
-# * Prefer any AES-GCM and ChaCha20 over any AES-CBC for better
-# performance and security
-# * Then Use HIGH cipher suites as a fallback
-# * Disable NULL authentication, NULL encryption, 3DES and MD5 MACs
-# for security reasons
-_DEFAULT_CIPHERS = (
- 'TLS13-AES-256-GCM-SHA384:TLS13-CHACHA20-POLY1305-SHA256:'
- 'TLS13-AES-128-GCM-SHA256:'
- 'ECDH+AESGCM:ECDH+CHACHA20:DH+AESGCM:DH+CHACHA20:ECDH+AES256:DH+AES256:'
- 'ECDH+AES128:DH+AES:ECDH+HIGH:DH+HIGH:RSA+AESGCM:RSA+AES:RSA+HIGH:'
- '!aNULL:!eNULL:!MD5:!3DES'
- )
-
-# Restricted and more secure ciphers for the server side
-# This list has been explicitly chosen to:
-# * TLS 1.3 ChaCha20 and AES-GCM cipher suites
-# * Prefer cipher suites that offer perfect forward secrecy (DHE/ECDHE)
-# * Prefer ECDHE over DHE for better performance
-# * Prefer AEAD over CBC for better performance and security
-# * Prefer AES-GCM over ChaCha20 because most platforms have AES-NI
-# * Prefer any AES-GCM and ChaCha20 over any AES-CBC for better
-# performance and security
-# * Then Use HIGH cipher suites as a fallback
-# * Disable NULL authentication, NULL encryption, MD5 MACs, DSS, RC4, and
-# 3DES for security reasons
-_RESTRICTED_SERVER_CIPHERS = (
- 'TLS13-AES-256-GCM-SHA384:TLS13-CHACHA20-POLY1305-SHA256:'
- 'TLS13-AES-128-GCM-SHA256:'
- 'ECDH+AESGCM:ECDH+CHACHA20:DH+AESGCM:DH+CHACHA20:ECDH+AES256:DH+AES256:'
- 'ECDH+AES128:DH+AES:ECDH+HIGH:DH+HIGH:RSA+AESGCM:RSA+AES:RSA+HIGH:'
- '!aNULL:!eNULL:!MD5:!DSS:!RC4:!3DES'
-)
+_RESTRICTED_SERVER_CIPHERS = _DEFAULT_CIPHERS
CertificateError = SSLCertVerificationError
@@ -393,8 +353,6 @@ class SSLContext(_SSLContext):
def __new__(cls, protocol=PROTOCOL_TLS, *args, **kwargs):
self = _SSLContext.__new__(cls, protocol)
- if protocol != _SSLv2_IF_EXISTS:
- self.set_ciphers(_DEFAULT_CIPHERS)
return self
def __init__(self, protocol=PROTOCOL_TLS):
@@ -530,8 +488,6 @@ def create_default_context(purpose=Purpose.SERVER_AUTH, *, cafile=None,
# verify certs and host name in client mode
context.verify_mode = CERT_REQUIRED
context.check_hostname = True
- elif purpose == Purpose.CLIENT_AUTH:
- context.set_ciphers(_RESTRICTED_SERVER_CIPHERS)
if cafile or capath or cadata:
context.load_verify_locations(cafile, capath, cadata)
diff --git a/Lib/test/test_ssl.py b/Lib/test/test_ssl.py
index fdf727f788ca..6a8bf0ef7be4 100644
--- a/Lib/test/test_ssl.py
+++ b/Lib/test/test_ssl.py
@@ -18,6 +18,7 @@
import weakref
import platform
import functools
+import sysconfig
try:
import ctypes
except ImportError:
@@ -30,7 +31,7 @@
HOST = support.HOST
IS_LIBRESSL = ssl.OPENSSL_VERSION.startswith('LibreSSL')
IS_OPENSSL_1_1 = not IS_LIBRESSL and ssl.OPENSSL_VERSION_INFO >= (1, 1, 0)
-
+PY_SSL_DEFAULT_CIPHERS = sysconfig.get_config_var('PY_SSL_DEFAULT_CIPHERS')
def data_file(*name):
return os.path.join(os.path.dirname(__file__), *name)
@@ -936,6 +937,19 @@ def test_ciphers(self):
with self.assertRaisesRegex(ssl.SSLError, "No cipher can be selected"):
ctx.set_ciphers("^$:,;?*'dorothyx")
+ @unittest.skipUnless(PY_SSL_DEFAULT_CIPHERS == 1,
+ "Test applies only to Python default ciphers")
+ def test_python_ciphers(self):
+ ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
+ ciphers = ctx.get_ciphers()
+ for suite in ciphers:
+ name = suite['name']
+ self.assertNotIn("PSK", name)
+ self.assertNotIn("SRP", name)
+ self.assertNotIn("MD5", name)
+ self.assertNotIn("RC4", name)
+ self.assertNotIn("3DES", name)
+
@unittest.skipIf(ssl.OPENSSL_VERSION_INFO < (1, 0, 2, 0, 0), 'OpenSSL too old')
def test_get_ciphers(self):
ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
diff --git a/Misc/NEWS.d/next/Library/2018-01-28-22-40-05.bpo-31429.qNt8rQ.rst b/Misc/NEWS.d/next/Library/2018-01-28-22-40-05.bpo-31429.qNt8rQ.rst
new file mode 100644
index 000000000000..1ac537b07aaf
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-01-28-22-40-05.bpo-31429.qNt8rQ.rst
@@ -0,0 +1,4 @@
+The default cipher suite selection of the ssl module now uses a blacklist
+approach rather than a hard-coded whitelist. Python no longer re-enables
+ciphers that have been blocked by OpenSSL security update. Default cipher
+suite selection can be configured on compile time.
diff --git a/Modules/_ssl.c b/Modules/_ssl.c
index ec8c8af54bdd..7545e91babdb 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -234,6 +234,31 @@ SSL_SESSION_get_ticket_lifetime_hint(const SSL_SESSION *s)
#endif /* OpenSSL < 1.1.0 or LibreSSL */
+/* Default cipher suites */
+#ifndef PY_SSL_DEFAULT_CIPHERS
+#define PY_SSL_DEFAULT_CIPHERS 1
+#endif
+
+#if PY_SSL_DEFAULT_CIPHERS == 0
+ #ifndef PY_SSL_DEFAULT_CIPHER_STRING
+ #error "Py_SSL_DEFAULT_CIPHERS 0 needs Py_SSL_DEFAULT_CIPHER_STRING"
+ #endif
+#elif PY_SSL_DEFAULT_CIPHERS == 1
+/* Python custom selection of sensible ciper suites
+ * DEFAULT: OpenSSL's default cipher list. Since 1.0.2 the list is in sensible order.
+ * !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
+ */
+ #define PY_SSL_DEFAULT_CIPHER_STRING "DEFAULT:!aNULL:!eNULL:!MD5:!3DES:!DES:!RC4:!IDEA:!SEED:!aDSS:!SRP:!PSK"
+#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
+#else
+ #error "Unsupported PY_SSL_DEFAULT_CIPHERS"
+#endif
+
enum py_ssl_error {
/* these mirror ssl.h */
@@ -2873,7 +2898,12 @@ _ssl__SSLContext_impl(PyTypeObject *type, int proto_version)
/* A bare minimum cipher list without completely 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");
+#if PY_SSL_DEFAULT_CIPHERS == 2
+ /* stick to OpenSSL's default settings */
+ result = 1;
+#else
+ result = SSL_CTX_set_cipher_list(ctx, PY_SSL_DEFAULT_CIPHER_STRING);
+#endif
} else {
/* SSLv2 needs MD5 */
result = SSL_CTX_set_cipher_list(ctx, "HIGH:!aNULL:!eNULL");
@@ -5430,6 +5460,9 @@ PyInit__ssl(void)
(PyObject *)&PySSLSession_Type) != 0)
return NULL;
+ PyModule_AddStringConstant(m, "_DEFAULT_CIPHERS",
+ PY_SSL_DEFAULT_CIPHER_STRING);
+
PyModule_AddIntConstant(m, "SSL_ERROR_ZERO_RETURN",
PY_SSL_ERROR_ZERO_RETURN);
PyModule_AddIntConstant(m, "SSL_ERROR_WANT_READ",
diff --git a/configure b/configure
index f94d16bf0b0c..caa86677ea11 100755
--- a/configure
+++ b/configure
@@ -840,6 +840,7 @@ enable_big_digits
with_computed_gotos
with_ensurepip
with_openssl
+with_ssl_default_suites
'
ac_precious_vars='build_alias
host_alias
@@ -1538,6 +1539,11 @@ Optional Packages:
--with(out)-ensurepip=[=upgrade]
"install" or "upgrade" using bundled pip
--with-openssl=DIR root of the OpenSSL directory
+ --with-ssl-default-suites=[python|openssl|STRING]
+ 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
Some influential environment variables:
MACHDEP name for machine-dependent library files
@@ -16931,6 +16937,48 @@ $as_echo "#define HAVE_X509_VERIFY_PARAM_SET1_HOST 1" >>confdefs.h
LIBS="$save_LIBS"
fi
+# ssl module default cipher suite string
+
+
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for --with-ssl-default-suites" >&5
+$as_echo_n "checking for --with-ssl-default-suites... " >&6; }
+
+# Check whether --with-ssl-default-suites was given.
+if test "${with_ssl_default_suites+set}" = set; then :
+ withval=$with_ssl_default_suites;
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $withval" >&5
+$as_echo "$withval" >&6; }
+case "$withval" in
+ python)
+ $as_echo "#define PY_SSL_DEFAULT_CIPHERS 1" >>confdefs.h
+
+ ;;
+ openssl)
+ $as_echo "#define PY_SSL_DEFAULT_CIPHERS 2" >>confdefs.h
+
+ ;;
+ *)
+ $as_echo "#define PY_SSL_DEFAULT_CIPHERS 0" >>confdefs.h
+
+ cat >>confdefs.h <<_ACEOF
+#define PY_SSL_DEFAULT_CIPHER_STRING "$withval"
+_ACEOF
+
+ ;;
+esac
+
+else
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: python" >&5
+$as_echo "python" >&6; }
+$as_echo "#define PY_SSL_DEFAULT_CIPHERS 1" >>confdefs.h
+
+
+fi
+
+
+
# generate output files
ac_config_files="$ac_config_files Makefile.pre Misc/python.pc Misc/python-config.sh"
diff --git a/configure.ac b/configure.ac
index 65248630b6e1..af10a6d3cad5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -5497,6 +5497,43 @@ if test "$have_openssl" = yes; then
LIBS="$save_LIBS"
fi
+# ssl module default cipher suite string
+AH_TEMPLATE(PY_SSL_DEFAULT_CIPHERS,
+ [Default cipher suites list for ssl module.
+ 1: Python's preferred selection, 2: leave OpenSSL defaults untouched, 0: custom string])
+AH_TEMPLATE(PY_SSL_DEFAULT_CIPHER_STRING,
+ [Cipher suite string for PY_SSL_DEFAULT_CIPHERS=0]
+)
+
+AC_MSG_CHECKING(for --with-ssl-default-suites)
+AC_ARG_WITH(ssl-default-suites,
+ AS_HELP_STRING([--with-ssl-default-suites=@<:@python|openssl|STRING@:>@],
+ [Override default cipher suites string,
+ python: use Python's preferred selection (default),
+ openssl: leave OpenSSL's defaults untouched,
+ STRING: use a custom string,
+ PROTOCOL_SSLv2 ignores the setting]),
+[
+AC_MSG_RESULT($withval)
+case "$withval" in
+ python)
+ AC_DEFINE(PY_SSL_DEFAULT_CIPHERS, 1)
+ ;;
+ openssl)
+ AC_DEFINE(PY_SSL_DEFAULT_CIPHERS, 2)
+ ;;
+ *)
+ AC_DEFINE(PY_SSL_DEFAULT_CIPHERS, 0)
+ AC_DEFINE_UNQUOTED(PY_SSL_DEFAULT_CIPHER_STRING, "$withval")
+ ;;
+esac
+],
+[
+AC_MSG_RESULT(python)
+AC_DEFINE(PY_SSL_DEFAULT_CIPHERS, 1)
+])
+
+
# generate output files
AC_CONFIG_FILES(Makefile.pre Misc/python.pc Misc/python-config.sh)
AC_CONFIG_FILES([Modules/ld_so_aix], [chmod +x Modules/ld_so_aix])
diff --git a/pyconfig.h.in b/pyconfig.h.in
index a18e3ca854e6..a0efff9777d2 100644
--- a/pyconfig.h.in
+++ b/pyconfig.h.in
@@ -1314,6 +1314,13 @@
/* Define to printf format modifier for Py_ssize_t */
#undef PY_FORMAT_SIZE_T
+/* Default cipher suites list for ssl module. 1: Python's preferred selection,
+ 2: leave OpenSSL defaults untouched, 0: custom string */
+#undef PY_SSL_DEFAULT_CIPHERS
+
+/* Cipher suite string for PY_SSL_DEFAULT_CIPHERS=0 */
+#undef PY_SSL_DEFAULT_CIPHER_STRING
+
/* Define to emit a locale compatibility warning in the C locale */
#undef PY_WARN_ON_C_LOCALE
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