lib/controllers/slack/botkit/index.js
function secureWebhookEndpoints() {
    var authenticationMiddleware = require(__dirname + '/middleware/slack_authentication.js');
    // convert a variable argument list to an array, drop the webserver argument
    var tokens = Array.prototype.slice.call(arguments);
    var webserver = tokens.shift();

    slack_botkit.logger.info(
      '** Requiring token authentication for webhook endpoints for Slash commands ' +
      'and outgoing webhooks; configured ' + tokens.length + ' token(s)'
    );

    webserver.use(authenticationMiddleware(tokens));
  }
function secureWebhookEndpoints() {
    var authenticationMiddleware = require(__dirname + '/middleware/slack_authentication.js');
    // convert a variable argument list to an array, drop the webserver argument
    var tokens = Array.prototype.slice.call(arguments);
    var webserver = tokens.shift();

    slack_botkit.logger.info(
      '** Requiring token authentication for webhook endpoints for Slash commands ' +
      'and outgoing webhooks; configured ' + tokens.length + ' token(s)'
    );

    webserver.use(authenticationMiddleware(tokens));
  }
adds the webhook authentication middleware module to the webserver