bpo-40695: Limit hashlib builtin hash fallback (GH-20259)

:mod:`hashlib` no longer falls back to builtin hash implementations when
OpenSSL provides a hash digest and the algorithm is blocked by security
policy.

Signed-off-by: Christian Heimes <[email protected]>
(cherry picked from commit 4cc2f93)

Co-authored-by: Christian Heimes <[email protected]>

Author: miss-islington

Lib/hashlib.py

@@ -127,8 +127,9 @@ def __get_openssl_constructor(name):
         # SHA3/shake are available in OpenSSL 1.1.1+
         f = getattr(_hashlib, 'openssl_' + name)
         # Allow the C module to raise ValueError.  The function will be
-        # defined but the hash not actually available thanks to OpenSSL.
-        f()
+        # defined but the hash not actually available.  Don't fall back to
+        # builtin if the current security policy blocks a digest, bpo#40695.
+        f(usedforsecurity=False)
         # Use the C function directly (very fast)
         return f
     except (AttributeError, ValueError):

Misc/NEWS.d/next/Library/2020-05-20-13-03-28.bpo-40695.lr4aIS.rst

@@ -0,0 +1,3 @@
+:mod:`hashlib` no longer falls back to builtin hash implementations when
+OpenSSL provides a hash digest and the algorithm is blocked by security
+policy.