code: def _authenticate_x509(credentials, sock_info):
      
      query = SON([('authenticate', 1),
                   ('mechanism', 'MONGODB-X509')])
      if credentials.username is not None:
          query['user'] = credentials.username
      elif sock_info.max_wire_version < 5:
          raise ConfigurationError(
              "A username is required for MONGODB-X509 authentication "
              "when connected to MongoDB versions older than 3.4.")
      sock_info.command('$external', query)
docs: Authenticate using MONGODB-X509.