CVE-2019-3875: Improper Certificate Validation

The X.509 authenticator supports the verification of client certificates through the CRL, where the CRL list can be obtained from the URL provided in the certificate itself (CDP) or through the separately configured path. The CRL are often available over the network through unsecured protocols (http or ldap) and hence the caller should verify the signature and possibly the certification path. Keycloak currently does not validate signatures on CRL, which can result in a possibility of various attacks like man-in-the-middle.