verdaccio/test/unit/partials/plugin/authenticate.fail.js

import { errorUtils } from '@verdaccio/core';

module.exports = function ( ) {
  return {
    authenticate( user, pass, callback ) {
			// we return an 500 error, the second argument must be false.
			// https://verdaccio.org/docs/en/dev-plugins#onerror
      callback(errorUtils.getInternalError(), false);
    }
  };
};