A reader of this blog, tjaffri, raised two questions:

  1. refresh_tokens did not work with Alexa (and Auth0).
  2. How to generate access_tokens for testing.

Refreshing access_tokens in Alexa

In my original prototype, I did not configure Alexa to request refresh_tokens so I didn’t know for sure, but I assumed it would since all this is based on “standards” (OpenID Connect/OAuth2).

Naturally, it failed. The reason it failed was that Auth0 was not returning the expires_in attribute in the refresh response body.

The token exchange endpoint is used for both code and refresh requests:

The difference between the two is is just the grant_type parameter in the body:

POST
https://{your tenant}.auth0.com/oauth/token
content-type: application/json
{ 
  "grant_type": "refresh_token",
  "client_id": "{CLIENT_ID}",
  "client_secret": "{CLIENT_SECRET}",
  "refresh_token": "{refresh_token}"
}

In a code exchange request, the response looks like this:

{ 
  "access_token": "{THE ACCESS TOKEN}",
  "id_token": "{THE ID TOKEN}",
  "expires_in": 120
}

Alexa uses the expires_in value to decide whether a new token is required or not.

Notice that the access_token is generally an opaque thing. Only to be consumed and interpreted by the API. The fact that in Auth0’s case it is actually a JWT (and therefore “decodable”), is not relevant. Alexa doesn’t know how to extract that information. Thus the expires_in parameter.

The reason it didn’t work is simple: a bug. Auth0 omitted the expires_in in the refresh response, and that broke Alexa (not entirely though, it just didn’t refresh the token).

expires_in is technically RECOMMENDED not required. But it is required in Alexa. This of course is not super clear in the documentation.

A fix is now ready and the flow will work as expected once it is live sometime next week.

Reminder: refresh_token is sent by Auth0 when you add the offline_access scope to the authorization request.

What to do in the meantime?

There’s a workaround you can try if you cannot wait until the fix is in production. You can proxy the token exchange request. And here comes Webtask for the rescue:

The code would look like this:

var Webtask = require('webtask-tools');
var request = require('request');
var app = new (require('express'))();

app.post('/token', function(req, res) {
  console.log(req.webtaskContext.data);
  
  var grant_type = req.webtaskContext.data.grant_type;
  var client_id = req.webtaskContext.data.client_id;
  var client_secret = req.webtaskContext.data.client_secret;
  var redirect_uri = req.webtaskContext.data.redirect_uri;
  
  var options = { 
                  method: 'POST',
                  url: 'https://{YOUR AUTH0 ACCOUNT}.auth0.com/oauth/token',
                  json: { 
                     'grant_type': grant_type,
                     'client_id': client_id,
                     'client_secret': client_secret,
                     'code': code,
                     'redirect_uri': redirect_uri
                  }
                };

  if(grant_type == 'authorization_code'){
      options.json.code = req.webtaskContext.data.code;
  } else {   
    if(grant_type == 'refresh_token')
    {
      options.json.refresh_token=req.webtaskContext.data.refresh_token;
    }
  }

  request(options, function(err, res, body) {
    console.log('Error', err);
    if(err) return res.sendStatus(500);
    if(options.json.refresh_token){  
      //Add the missing expires_in for refresh token flow
      body.expires_in = 120;
      res.json(body);
    }
  });
});

module.exports = Webtask.fromExpress(app);

Then in your Alexa configuration, you use the Webtask URL as the token exchange endpoint.

Getting Test access_tokens for your API

This is an easy one. You just create a new Client (e.g. your test runner). Then you can use client credentials flow to obtain a token.

  1. Register a new Client in Auth0 (you can specify “Non Interactive Client”).
  2. Then on the API section, click on Clients and authorize it to request tokens with the scopes you need.
  3. Use the client credentials flow to request a token for this API (with the given scopes).
curl --request POST \
  --url https://{YOUR AUTH0 ACCOUNT}.auth0.com/oauth/token \
  --header 'content-type: application/json' \
  --data '{"client_id":"{THE TEST CLIENT ID}","client_secret":"{THE TEST CLIENT SECRET}","audience":"https://whatshouldiwear","grant_type":"client_credentials"}'

Auth0 will send you back an access_token you can use with your test runner. When you are done, you can simply delete the Client. No changes will happen on the API or anything.

Merry Christmas tjaffri!