Appeal to authority would be something like, "North Korea wasn't behind the attack because Schneier says so." To avoid an appeal to authority, you cite Schneier

Appeal to authority would be something like, "North Korea wasn't behind the attack because Schneier says so." To avoid an appeal to authority, you cite Schneier but make use of his arguments. "According to Schneier, North Korea wasn't behind the attack because if North Korea was behind the attack a different locale would have been used." The fallacy still applies when the authority gives an opinion on some topic they are knowledgeable about; the fallacy isn't about the relevance of the authority so much as the use of the appeal.