There is a type confusion vulnerability relating to X.400 address processing
inside an X.509 GeneralName. X.400 addresses were parsed as an ASN1_STRING but
the public structure definition for GENERAL_NAME incorrectly specified the type
of the x400Address field as ASN1_TYPE. This field is subsequently interpreted by
the OpenSSL function GENERAL_NAME_cmp as an ASN1_TYPE rather than an
ASN1_STRING.
When CRL checking is enabled (i.e. the application sets the
X509_V_FLAG_CRL_CHECK flag), this vulnerability may allow an attacker to pass
arbitrary pointers to a memcmp call, enabling them to read memory contents or
enact a denial of service. In most cases, the attack requires the attacker to
provide both the certificate chain and CRL, neither of which need to have a
valid signature. If the attacker only controls one of these inputs, the other
input must already contain an X.400 address as a CRL distribution point, which
is uncommon. As such, this vulnerability is most likely to only affect
applications which have implemented their own functionality for retrieving CRLs
over a network.