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Issue #21013: Enhance ssl.create_default_context() for server side contexts

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Closes #21013 by modfying ssl.create_default_context() to:

* Move the restricted ciphers to only apply when using
  ssl.Purpose.CLIENT_AUTH. The major difference between restricted and not
  is the lack of RC4 in the restricted. However there are servers that exist
  that only expose RC4 still.
* Switches the default protocol to ssl.PROTOCOL_SSLv23 so that the context
  will select TLS1.1 or TLS1.2 if it is available.
* Add ssl.OP_NO_SSLv3 by default to continue to block SSL3.0 sockets
* Add ssl.OP_SINGLE_DH_USE and ssl.OP_SINGLE_ECDG_USE to improve the security
  of the perfect forward secrecy
* Add ssl.OP_CIPHER_SERVER_PREFERENCE so that when used for a server side
  socket the context will prioritize our ciphers which have been carefully
  selected to maximize security and performance.
* Documents the failure conditions when a SSL3.0 connection is required so
  that end users can more easily determine if they need to unset
  ssl.OP_NO_SSLv3.
This commit is contained in:
Donald Stufft 2014-03-23 19:05:28 -04:00
parent 553e108fce
commit 6a2ba94908
4 changed files with 70 additions and 16 deletions
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30
Lib/ssl.py
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@ -179,7 +179,7 @@ _DEFAULT_CIPHERS = (
'DH+RC4:RSA+RC4:!aNULL:!eNULL:!MD5'
)
# Restricted and more secure ciphers
# Restricted and more secure ciphers for the server side
# This list has been explicitly chosen to:
# * Prefer cipher suites that offer perfect forward secrecy (DHE/ECDHE)
# * Prefer ECDHE over DHE for better performance
@ -188,7 +188,7 @@ _DEFAULT_CIPHERS = (
# * Then Use 3DES as fallback which is secure but slow
# * Disable NULL authentication, NULL encryption, MD5 MACs, DSS, and RC4 for
# security reasons
_RESTRICTED_CIPHERS = (
_RESTRICTED_SERVER_CIPHERS = (
'ECDH+AESGCM:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:ECDH+HIGH:'
'DH+HIGH:ECDH+3DES:DH+3DES:RSA+AESGCM:RSA+AES:RSA+HIGH:RSA+3DES:!aNULL:'
'!eNULL:!MD5:!DSS:!RC4'
@ -404,17 +404,35 @@ def create_default_context(purpose=Purpose.SERVER_AUTH, *, cafile=None,
"""
if not isinstance(purpose, _ASN1Object):
raise TypeError(purpose)
context = SSLContext(PROTOCOL_TLSv1)
context = SSLContext(PROTOCOL_SSLv23)
# 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)
# disallow ciphers with known vulnerabilities
context.set_ciphers(_RESTRICTED_CIPHERS)
# verify certs and host name in client mode
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:
context.load_verify_locations(cafile, capath, cadata)
elif context.verify_mode != CERT_NONE: