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A X509 cert with a registered id general name in subject alternative name causes a SystemError: error return without exception set. This prevents host name validation of certs with a registered id.
>>> import _ssl
>>> _ssl._test_decode_cert('rid.pem')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
SystemError: error return without exception set
The problem is caused by a bug in OpenSSL's print function for general names. Python's _get_peer_alt_names() uses GENERAL_NAME_print() to print GEN_IPADD, GEN_RID and others into a buffer. The buffer is then split at ':' into two strings. This works for all fields except for GEN_RID because OpenSSL doesn't put a ':' after 'Registered ID', https://github.com/openssl/openssl/blob/master/crypto/x509v3/v3_alt.c#L183 . _get_peer_alt_names() fails and returns NULL without setting a proper exception.
It looks like we haven't had tests for GEN_RID as well as some other field types.
Related Red Hat bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1364268 

ping

I can't comment on this, as I don't even know what a "registered id" is, sorry :-/

A GEN_RID is an OID plus some opaque data. It's up to an application to understand an OID and interpret its data. The value of a GEN_RID can be as simple as an int or UTF-8 strings or as complex as a nested ASN.1 struct for Kerberos principals.
I have modified Lib/test/make_ssl_certs.py to include two GEN_RIDS: 
 otherName.1 = 1.2.3.4;UTF8:some other identifier
 otherName.2 = 1.3.6.1.5.2.2;SEQUENCE:princ_name
 [princ_name]
 realm = EXP:0, GeneralString:KERBEROS.REALM
 principal_name = EXP:1, SEQUENCE:principal_seq
 [principal_seq]
 name_type = EXP:0, INTEGER:1
 name_string = EXP:1, SEQUENCE:principals
 [principals]
 princ1 = GeneralString:username
1.3.6.1.5.2.2 is the OID for Kerberos public key init (pkinit), used for e.g. FAST pre-auth and SmartCard authentication.

New changeset 9bbf0b31da48 by Christian Heimes in branch '3.5':
Issue #27691: Fix ssl module's parsing of GEN_RID subject alternative name fields in X.509 certs.
https://hg.python.org/cpython/rev/9bbf0b31da48
New changeset 2b9af57af3e4 by Christian Heimes in branch 'default':
Issue #27691: Fix ssl module's parsing of GEN_RID subject alternative name fields in X.509 certs.
https://hg.python.org/cpython/rev/2b9af57af3e4
New changeset 74805fd9e734 by Christian Heimes in branch '2.7':
Issue #27691: Fix ssl module's parsing of GEN_RID subject alternative name fields in X.509 certs.
https://hg.python.org/cpython/rev/74805fd9e734