DDOS Vulnerability Fix - Secure Facebook Webhook

facebook_bot.js

@@ -76,6 +76,11 @@ if (!process.env.verify_token) {
     process.exit(1);
 }
 
+if (!process.env.app_secret) {
+    console.log('Error: Specify app_secret in environment');
+    process.exit(1);
+}
+
 var Botkit = require('./lib/Botkit.js');
 var os = require('os');
 var commandLineArgs = require('command-line-args');
@@ -99,6 +104,8 @@ var controller = Botkit.facebookbot({
     log: true,
     access_token: process.env.page_token,
     verify_token: process.env.verify_token,
+    app_secret: process.env.app_secret
+    validate_requests: true, // Refuse any requests that don't come from FB on your receive webhook, must provide FB_APP_SECRET in environment variables
 });
 
 var bot = controller.spawn({

lib/Facebook.js

@@ -2,6 +2,7 @@ var Botkit = require(__dirname + '/CoreBot.js');
 var request = require('request');
 var express = require('express');
 var bodyParser = require('body-parser');
+var crypto = require('crypto')
 
 function Facebookbot(configuration) {
 
@@ -355,6 +356,14 @@ function Facebookbot(configuration) {
         facebook_botkit.config.port = port;
 
         facebook_botkit.webserver = express();
+
+        // Validate that requests come from facebook, and abort on validation errors
+        if (facebook_botkit.config.validate_requests === true) {
+            // Load verify middleware just for post route on our receive webhook, and catch any errors it might throw to prevent the request from being parsed further.
+            facebook_botkit.webserver.post('/facebook/receive', bodyParser.json({verify: verifyRequest}))
+            facebook_botkit.webserver.use(abortOnValidationError)
+        }
+
         facebook_botkit.webserver.use(bodyParser.json());
         facebook_botkit.webserver.use(bodyParser.urlencoded({ extended: true }));
         facebook_botkit.webserver.use(express.static(static_dir));
@@ -475,6 +484,34 @@ function Facebookbot(configuration) {
         }
     };
 
+    // Verifies the SHA1 signature of the raw request payload before bodyParser parses it
+    // Will abort parsing if signature is invalid, and pass a generic error to response
+    function verifyRequest(req, res, buf, encoding) {
+      var expected = req.headers['x-hub-signature'];
+      var calculated = getSignature(buf);
+      if (expected !== calculated) {
+        throw new Error("Invalid signature on incoming request");
+      } else {
+        facebook_botkit.debug('** X-Hub Verification successful!')
+      }
+    }
+
+    function getSignature(buf) {
+      var hmac = crypto.createHmac("sha1", facebook_botkit.config.app_secret);
+      hmac.update(buf, "utf-8");
+      return "sha1=" + hmac.digest("hex");
+    }
+
+    function abortOnValidationError(err, req, res, next) {
+      if (err) {
+        facebook_botkit.log('** Invalid X-HUB signature on incoming request!')
+        facebook_botkit.debug('** X-HUB Validation Error:', err)
+        res.status(400).send({ error: "Invalid signature." });
+      } else {
+        next();
+      }
+    }
+
     return facebook_botkit;
 };
 

readme-facebook.md

@@ -54,6 +54,15 @@ Since Facebook delivers messages via web hook, your application must be availabl
 
 When you are ready to go live, consider [LetsEncrypt.org](http://letsencrypt.org), a _free_ SSL Certificate Signing Authority which can be used to secure your website very quickly. It is fabulous and we love it.
 
+## Validate Requests - Secure your webhook!
+Facebook sends an X-HUB signature header with requests to your webhook. You can verify the requests are coming from Facebook by enabling `validate_requests: true` when creating your bot controller. This checks the sha1 signature of the incoming payload against your Facebook App Secret (which is seperate from your webhook's verify_token), preventing unauthorized access to your webhook. You must also pass your `app_secret` into your environment variables when running your bot.
+
+The Facebook App secret is available on the Overview page of your Facebook App's admin page. Click show to reveal it.
+
+```
+app_secret=abcdefg12345 page_token=123455abcd verify_token=VerIfY-tOkEn node facebook_bot.js
+```
+
 ## Facebook-specific Events
 
 Once connected to Facebook, bots receive a constant stream of events.