OAuth 2.0: Implicit Flow is Dead, Try PKCE Instead

There are a number of OAuth 2.0 flows that can be used in various scenarios. The Implicit flow was previously recommended for native, mobile, and browser-based apps to immediately grant the user an access token. In this post, we’ll learn why the Authorization Code flow (with PKCE) is the new standard for more secure authorization for these types of apps.

With the release of Postman v7.23, we announced support for Proof Key for Code Exchange, better known as PKCE (pronounced “pixy”). Let’s walk through a few of the common OAuth 2.0 flows in Postman before we get into why PKCE has become an IETF-recommended authorization flow.

“Security and privacy controls are ‘must-have’ features these days. When thinking about all the nuances of protecting resources and access tokens, enable only the grant types necessary and implement the strictest controls available.” –Daniel Katz, Senior Product Manager at Ping Identity

What is OAuth?

Related: What is API authentication?

When people talk about OAuth, they typically mean OAuth 2.0—an authorization framework that describes how unrelated services can grant access to resources. It’s an open standard used by apps, APIs, and other services over HTTPS.

One example of OAuth is when you log into a website and are prompted to log in using an unrelated website’s login. The other website authenticates you and gives you permission to access this website.

OAuth decouples authentication from authorization, by relying on a third party to grant an access token. Doing this reduces your attack surface since your client secret is not required to access certain resources.

“Many luxury cars today come with a valet key. It is a special key you give the parking attendant and unlike your regular key, will not allow the car to drive more than a mile or two. Some valet keys will not open the trunk, while others will block access to your onboard cell phone address book … OAuth is about giving access to your stuff without sharing your identity at all (or its secret parts).”  (OAuth.net)

OAuth 2.0 has different grant types for various scenarios. Here are a few of them.

• Password: for logging in with username and password
• Client credentials: for when a user is not present
• Authorization Code: for mobile and web apps
• Implicit: historically used for single-page JavaScript apps where secrets cannot be securely stored

Let’s take a look at two commonly used grant types, Authorization Code and Implicit.

What is the Authorization Code flow for OAuth?

One widely used grant type is the Authorization Code flow. JSON web token (JWT) is one standard that uses this type of grant.

Instead of requesting tokens directly from your API, the Authorization Code flow protects a client secret by redirecting a request for a token through an Authorization Server.

The Authorization Server authenticates a user and approves their access to a resource by providing a temporary authorization code. A token can then be requested using your credentials along with this authorization code.

The authorization code offers an additional layer of security. However, this grant requires sending the client secret to request the access token. The OAuth service provider can then verify the request is coming from a legitimate client, instead of an attacker who has intercepted the authorization code.

What is the Implicit flow for OAuth?

The Implicit grant was previously recommended for native and browser-based applications, whose client secrets cannot be revealed on the frontend:

• Native mobile apps, whose client secrets cannot be securely stored since decompiling the app will reveal them
• Browser-based apps, like single-page apps (SPAs), whose client secrets cannot be securely stored because the app’s source code is available to the browser

For these scenarios, the Implicit grant is a simplified Authorization Code flow that directly issues an access token without authenticating the client.

Note: Do not use the Implicit flow for authorization. Instead, use the Authorization Code flow (with PKCE) for your native, mobile, and browser-based apps.

What is PKCE?

“PKCE (Proof Key for Code Exchange) is an extension to the Authorization Code flow to prevent certain attacks and to be able to securely perform the OAuth exchange from public clients.” (OAuth.net)

For native and browser-based JavaScript apps, it is now widely considered a best practice to use the Authorization Code flow with the PKCE extension, instead of the Implicit flow.

This flow is like the regular Authorization Code flow, except PKCE replaces the client secret used in the standard Authorization Code flow with a one-time code challenge. This means the client app doesn’t have to store a client secret.

How to use PKCE in Postman

Earlier, we saw another layer of security for OAuth public clients using the following elements:

• Code verifier: a cryptographically random string used to correlate the authorization request to the token request
• Code challenge: derived from the code verifier sent in the authorization request, to be verified against later
• Code challenge method: what was used to derive code challenge

Now we’re going to set up Authorization Code flow (with PKCE) in Postman.

1. In Postman, under the Authorization tab of any request, select OAuth 2.0.
2. Click Get New Access Token.
3. Select a Grant Type of Authorization Code (With PKCE).
4. The Code Challenge Method can be either SHA-256 or Plain.
5. You can also optionally provide a custom Code Verifier.

Click Request Token and walk through the authorization process to generate a new token. The access token can then be used according to your specific API’s documentation.

Try it out in Postman

If you want to try out some examples in Postman, check out our template walking through various OAuth 2.0 flows.

Additional Resources

If you work with native or browser-based applications, the PKCE extension to the Authorization Code flow enables a more secure OAuth exchange from public clients.

If you’re new to the world of OAuth and PKCE, check out these helpful resources to get started:

• OAuth 2 Simplified from Aaron Parecki
• PKCE Example on the OAuth 2.0 Playground from Okta
• Call API using Authorization Code flow with PKCE from Auth0