Python should no longer attempt to verify hostname and ip addresses itself. OpenSSL 1.0.2 and newer is able to verify hostname and IP addresses itself. The new APIs are properly hooked into chain validation step. Hostname matching implements RFC 6125. CN matching and partial wildcards can be tuned with additional. The API is documented here: https://www.openssl.org/docs/man1.0.2/crypto/X509_VERIFY_PARAM_set1_host.html . X509_VERIFY_PARAM_set1_host is available since OpenSSL 1.0.2. LibreSSL 2.5.3+ implement the proper bits and pieces, too.
Why should we use OpenSSL rather than matching hostnames ourselves?
In the past, OpenSSL did not contain any code to perform host name
matching. Application were required to role their own implementation.
This caused code duplication and various security issues, because
it is far from trivial to cover all edge cases. Python had multiple
security issues just caused by incorrect or buggy hostname matching:
Until Python 3.2 and 2.7.9, the ssl module was not capable of
performing host name matching. ssl.match_hostname() was
introduced in 3.2.0 and later back-ported to 2.7.9.
Issue bpo-12000: Subject CN was ignored when a subject alternative
name extension (SAN) was present without dNSName entries, thus
violating RFC 2818.
CVE-2013-2099: Multiple wildcard characters could be abused
for Denial-of-Service attack in the re module.
Issue bpo-17997: RFC 2818 was superseded by RFC 6125, which no longer
allows multiple wildcard characters. Wildcards are only supported
in the left-most label.
Issue bpo-17997: ssl.match_hostname() did not implement partial
wildcards of international domain names correctly.
Issue bpo-18709: The ssl module used an inappropriate OpenSSL function
to convert host names from ASN.1 to strings. A host name with an
embedded NULL byte could be abused to trick validation.
Issue bpo-17305: The ssl module does not handle IDNA 2008-encoded
host names correctly. It converts from IDN A-label (ASCII
compatible encoding) to IDN U-label (unicode) with Python's idna
encoding, which is IDNA 2003-only.
Issue bpo-30141: The host name is not verified when a SSLSocket is
created with do_handshake_on_connect=False and the application
causes an implicit handshake w/o calling do_handshake() explicitly.
A SSLSocket performs host name matching after the handshake and
during the handshake. In case of an invalid host name, a client
is suppose to abort the connection with appropriate TLS alert.
This causes two problem. For one the server is not informed about
a problem with the certificate. Also an invalid host name does not
prevent the client from sending a TLS client authentication
cert to a malicious server. The cert typically contains personal
information like username and department.