gh-107361: strengthen default SSL context flags (#112389)

This adds `VERIFY_X509_STRICT` to make the default
SSL context perform stricter (per RFC 5280) validation, as well
as `VERIFY_X509_PARTIAL_CHAIN` to enforce more standards-compliant
path-building behavior.

As part of this changeset, I had to tweak `make_ssl_certs.py`
slightly to emit 5280-conforming CA certs. This changeset includes
the regenerated certificates after that change.

Signed-off-by: William Woodruff <[email protected]>
Co-authored-by: Victor Stinner <[email protected]>

Author: woodruffw

.gitattributes

@@ -83,6 +83,8 @@ Include/opcode_ids.h                                generated
 Include/token.h                                     generated
 Lib/_opcode_metadata.py                             generated
 Lib/keyword.py                                      generated
+Lib/test/certdata/*.pem                             generated
+Lib/test/certdata/*.0                               generated
 Lib/test/levenshtein_examples.json                  generated
 Lib/test/test_stable_abi_ctypes.py                  generated
 Lib/token.py                                        generated

Doc/library/ssl.rst

@@ -151,6 +151,12 @@ purposes.
    variable :envvar:`SSLKEYLOGFILE` is set, :func:`create_default_context`
    enables key logging.
 
+   The default settings for this context include
+   :data:`VERIFY_X509_PARTIAL_CHAIN` and :data:`VERIFY_X509_STRICT`.
+   These make the underlying OpenSSL implementation behave more like
+   a conforming implementation of :rfc:`5280`, in exchange for a small
+   amount of incompatibility with older X.509 certificates.
+
    .. note::
       The protocol, options, cipher and other settings may change to more
       restrictive values anytime without prior deprecation.  The values
@@ -172,6 +178,15 @@ purposes.
          ctx = ssl.create_default_context(Purpose.CLIENT_AUTH)
          ctx.options &= ~ssl.OP_NO_SSLv3
 
+   .. note::
+      This context enables :data:`VERIFY_X509_STRICT` by default, which
+      may reject pre-:rfc:`5280` or malformed certificates that the
+      underlying OpenSSL implementation otherwise would accept. While disabling
+      this is not recommended, you can do so using::
+
+         ctx = ssl.create_default_context()
+         ctx.verify_flags &= ~ssl.VERIFY_X509_STRICT
+
    .. versionadded:: 3.4
 
    .. versionchanged:: 3.4.4
@@ -194,6 +209,11 @@ purposes.
       :data:`PROTOCOL_TLS_SERVER` protocol instead of generic
       :data:`PROTOCOL_TLS`.
 
+   .. versionchanged:: 3.13
+
+      The context now uses :data:`VERIFY_X509_PARTIAL_CHAIN` and
+      :data:`VERIFY_X509_STRICT` in its default verify flags.
+
 
 Exceptions
 ^^^^^^^^^^

Doc/whatsnew/3.13.rst

@@ -62,7 +62,6 @@ Summary -- Release highlights
 .. This section singles out the most important changes in Python 3.13.
    Brevity is key.
 
-
 .. PEP-sized items next.
 
 Important deprecations, removals or restrictions:
@@ -192,6 +191,21 @@ Other Language Changes
 
   (Contributed by Arthur Tacca and Jason Zhang in :gh:`115957`.)
 
+* The :func:`ssl.create_default_context` API now includes
+  :data:`ssl.VERIFY_X509_PARTIAL_CHAIN` and :data:`ssl.VERIFY_X509_STRICT`
+  in its default flags.
+
+  .. note::
+
+   :data:`ssl.VERIFY_X509_STRICT` may reject pre-:rfc:`5280` or malformed
+   certificates that the underlying OpenSSL implementation otherwise would
+   accept. While disabling this is not recommended, you can do so using::
+
+      ctx = ssl.create_default_context()
+      ctx.verify_flags &= ~ssl.VERIFY_X509_STRICT
+
+  (Contributed by William Woodruff in :gh:`112389`.)
+
 New Modules
 ===========
 

Lib/ssl.py

@@ -704,6 +704,16 @@ def create_default_context(purpose=Purpose.SERVER_AUTH, *, cafile=None,
     else:
         raise ValueError(purpose)
 
+    # `VERIFY_X509_PARTIAL_CHAIN` makes OpenSSL's chain building behave more
+    # like RFC 3280 and 5280, which specify that chain building stops with the
+    # first trust anchor, even if that anchor is not self-signed.
+    #
+    # `VERIFY_X509_STRICT` makes OpenSSL more conservative about the
+    # certificates it accepts, including "disabling workarounds for
+    # some broken certificates."
+    context.verify_flags |= (_ssl.VERIFY_X509_PARTIAL_CHAIN |
+                             _ssl.VERIFY_X509_STRICT)
+
     if cafile or capath or cadata:
         context.load_verify_locations(cafile, capath, cadata)
     elif context.verify_mode != CERT_NONE:

Lib/test/certdata/allsans.pem

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 -----END CERTIFICATE-----

Lib/test/certdata/capath/b1930218.0

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 -----END CERTIFICATE-----