OAuth 2.0 in Practice: Building an OAuth Client

Ever stared at a “Sign in with Google” button and wondered about the magic happening behind the scenes? Or perhaps you’re building a fantastic new application, and you need it to securely talk to another service – say, to fetch a user’s photos from their favorite cloud storage or post an update to their social media, all without ever asking for their actual password for that service. If so, you’ve stumbled upon the domain of a quiet, powerful hero of the internet: OAuth 2.0.

Imagine you’re building “TaskMaster Supreme,” an app to rule all your to-do lists. You want it to sync with users’ Google Calendars. Asking users for their Google passwords? That’s a huge security no-no and, frankly, a bit scary for everyone involved! This is precisely where OAuth 2.0 steps in, acting as a trusted intermediary. This post is your guide to understanding and implementing an OAuth 2.0 client, turning your app from a standalone island into a connected, secure powerhouse.

What is OAuth 2.0? The Authorization Standard Behind Secure APIs

OAuth 2.0 is an authorization framework, not an authentication protocol. Think of it this way: authentication is about verifying who you are, while authorization is about determining what you’re allowed to do.

What is OAuth 2.0?

OAuth 2.0 is an open standard for access delegation, commonly used to grant websites or applications access to information on other websites on behalf of a user, without giving them the user’s passwords.

Also Read: OAuth 2.0 Overview

It’s like giving a valet key to a parking attendant. They can drive your car and park it (a specific, limited permission), but they can’t open your glove compartment or trunk. OAuth 2.0 allows users to grant third-party applications limited access to their resources on another service, without sharing their credentials.

To understand the OAuth 2.0 process, we need to know the key players:

1. Resource Owner: This is the end-user who owns the data (e.g., you, with your Google Calendar events).
2. Client: This is your application (e.g., “TaskMaster Supreme”) that wants to access the Resource Owner’s data.
3. Authorization Server: This server is responsible for authenticating the Resource Owner and issuing “access tokens” after getting their consent (e.g., Google’s authentication server).
4. Resource Server: This is the API server that hosts the protected resources (e.g., the Google Calendar API).

For a deep dive into the official specifications, the OAuth.net website is the ultimate source.

Beyond the Basics: Understanding the Power of OAuth 2.0

Implementing OAuth 2.0 might seem like an extra step, but the benefits are massive, both for you as a developer and for your users:

• Enhanced Security: The most significant win! Users never share their primary credentials (username/password for the other service) with your client application. This drastically reduces the risk of compromised accounts.
• Limited Access (Scopes): Your application only requests permission for specific actions or data types, known as “scopes.” For “TaskMaster Supreme,” you might only request permission to read and write calendar events, not access emails or contacts. This is the principle of least privilege in action.
• User Control: Users can grant or revoke access to your application at any time directly from the service provider’s settings (e.g., managing connected apps in Google Account settings).
• Standardization: OAuth 2.0 is a widely adopted industry standard, meaning many APIs you’ll want to integrate with will support it, and there are plenty of libraries and tools to help.

Real-World Use Cases:

•  Social Logins: “Sign in with Facebook,” “Log in with GitHub,” etc.
•  Third-Party App Integrations: A fitness app pulling workout data from a wearable device’s cloud service.
•  Cloud Service Access: A photo editing app accessing your photos stored in Dropbox or Google Photos.
•  API Access Management: Internal enterprise applications securely accessing microservices.

Understanding OAuth 2.0 Grant Types (Which Flow is for You?)

OAuth 2.0 defines several ways, called “grant types” or “flows,” for a client application to obtain an access token. The choice of grant type depends heavily on the type of client application (e.g., web server app, single-page app, mobile app, or machine-to-machine).

The most common and secure flow for web applications with a backend and for mobile apps is the Authorization Code Grant. Let’s walk through it with our “TaskMaster Supreme” example wanting to access a user’s “CloudCal” (a fictional calendar service).

1. User Initiates: The user in TaskMaster Supreme clicks “Connect to CloudCal.”
2. Redirection to Authorization Server: TaskMaster Supreme redirects the user’s browser to CloudCal’s Authorization Server. This redirect includes TaskMaster Supreme’s client_id and the scopes it’s requesting.
3. User Authenticates & Grants Consent: The user logs into CloudCal (if not already logged in) and sees a screen asking if they want to allow TaskMaster Supreme to access their calendar. They click “Allow.”
4. Authorization Code Issued: CloudCal’s Authorization Server redirects the user back to TaskMaster Supreme (to a pre-registered redirect_uri) with a temporary authorization_code.
5. Exchange Code for Token: TaskMaster Supreme’s backend server takes this authorization_code and, along with its client_id and client_secret (a confidential password for the app itself), makes a direct, secure request to CloudCal’s Authorization Server’s token endpoint.
6. Access Token Granted: If all is well, CloudCal’s Authorization Server issues an access_token (and often a refresh_token) back to TaskMaster Supreme.
7. Access Protected Resource: TaskMaster Supreme can now use this access_token to make API calls to CloudCal’s Resource Server (the Calendar API) to fetch or update the user’s calendar events.

While the Authorization Code Grant is prevalent, other grant types exist for different scenarios, such as:

•  Client Credentials Grant: Used for server-to-server communication where the client is the resource owner (e.g., a background service accessing its own API).
•  Implicit Grant (Legacy): Was used for browser-based (JavaScript) or mobile apps, but is now generally discouraged due to security concerns. PKCE (Proof Key for Code Exchange) with the Authorization Code Grant is preferred.
•  Resource Owner Password Credentials Grant (Discouraged): The client directly asks for the user’s username and password for the resource server. This should be avoided unless absolutely necessary and only for trusted first-party applications.

For a more exhaustive look at grant types

Implementing OAuth 2.0 Client: A Step-by-Step Guide

Alright, theory’s great, but let’s get practical. Here’s how you’d typically implement an OAuth 2.0 client using the Authorization Code Grant.

Step 1: Register Your Application

Before you write a single line of client code, you need to register your application (e.g., “TaskMaster Supreme”) with the service provider (e.g., Google, GitHub, Facebook, or your company’s internal OAuth server). This usually involves:

•  Going to their developer portal (e.g., Google Cloud Console, GitHub Developer Settings).
•  Providing an application name, description, website, logo, etc.
•  Specifying Redirect URI(s): These are the URLs the Authorization Server will redirect the user back to after they authorize (or deny) access. This is a critical security measure – the server will only redirect to registered URIs.
• Upon successful registration, you’ll receive:
  •  Client ID: A public identifier for your application.
  •  Client Secret: A confidential secret known only to your application and the Authorization Server. Guard this like a password! Never embed it in client-side code (like JavaScript in a browser or a mobile app’s code).

Step 2: Getting the Authorization Code

Your application needs to construct an authorization URL and redirect the user’s browser to it. The URL typically looks like this:

[https://auth-server.com/oauth/authorize?response_type=code&client_id=YOUR_CLIENT_ID&redirect_uri=YOUR_REGISTERED_REDIRECT_URI&scope=requested_scopes&state=UNIQUE_RANDOM_STRING](https://auth-server.com/oauth/authorize?response_type=code&client_id=YOUR_CLIENT_ID&redirect_uri=YOUR_REGISTERED_REDIRECT_URI&scope=requested_scopes&state=UNIQUE_RANDOM_STRING)

Let’s break down the query parameters:

•  response_type_code: Tells the Authorization Server you want an authorization code.
•  client_id: Your application’s Client ID.
•  redirect_uri: The URL where the user should be sent back after authorization. This must match one of the URIs you registered.
•  scope: A space-separated list of permissions your app is requesting (e.g., calendar.read calendar.write profile openid).
•  state: A random, unique, and non-guessable string that your application generates. You’ll store this (e.g., in the user’s session) and verify it when the user is redirected back. This helps prevent Cross-Site Request Forgery (CSRF) attacks.

When the user visits this URL, they’ll be prompted by the Authorization Server to log in (if they aren’t already) and approve the requested permissions for your application.

Step 3: Exchanging the Authorization Code for an Access Token

If the user approves, the Authorization Server redirects them back to your specified redirect_uri with the authorization_code and the state parameter appended as query strings:

[https://your-app.com/callback?code=AUTHORIZATION_CODE_FROM_SERVER&state=THE_SAME_UNIQUE_RANDOM_STRING](https://your-app.com/callback?code=AUTHORIZATION_CODE_FROM_SERVER&state=THE_SAME_UNIQUE_RANDOM_STRING)

Your application’s backend at /callback should:

1. Verify the state parameter: Compare the received state with the one you stored earlier. If they don’t match, abort the process as it could be a CSRF attack.
2. Exchange the code for an access token: Make a server-to-server POST request to the Authorization Server’s token endpoint. This request is not done through the user’s browser; it’s a backend call.

The POST request to the token endpoint (e.g., https://auth-server.com/oauth/token) will typically include:

•  grant_type=authorization_code
•  code=THE_AUTHORIZATION_CODE_YOU_RECEIVED
•  redirect_uri=YOUR_REGISTERED_REDIRECT_URI (must match the one used in Step 2)
•  client_id=YOUR_CLIENT_ID
•  client_secret=YOUR_CLIENT_SECRET (This is often sent via an Authorization: Basic <base64_encode(client_id:client_secret)> header or in the request body).

The Authorization Server will validate these details and, if everything checks out, respond with a JSON payload containing:

• access_token: The precious token your app will use to access the API.
• token_type: Usually “Bearer”.
• expires_in: The lifetime of the access token in seconds.
• refresh_token (optional but common): A long-lived token used to obtain new access tokens when the current one expires, without requiring user re-authentication.
• scope: The scopes actually granted (might be different from what you requested if the user denied some).

You should securely store the access_token (e.g., in a session or database associated with the user) and the refresh_token (if received, store it very securely, often encrypted).

Many programming languages have excellent libraries to simplify these steps, such as Authlib for Python or Spring Security OAuth for Java.

Step 4: Using the Access Token to Call the API

Now for the fun part! To make requests to the protected Resource Server (e.g., the CloudCal API), include the access_token in the Authorization header of your HTTP requests, using the “Bearer” scheme:

GET /api/calendar/events HTTP/1.1
Host: resource-server.com
Authorization: Bearer YOUR_ACCESS_TOKEN

The Resource Server will validate the token. If it’s valid and has the necessary scopes, it will process your request and return the data.

Step 5: Handling Token Expiration and Refreshing Tokens

Access tokens are intentionally short-lived for security reasons (e.g., 1 hour). When an access token expires, API calls using it will fail (usually with a 401 Unauthorized error).

If you received a refresh_token in Step 3, you can use it to get a new access_token without bothering the user again. Make another POST request to the Authorization Server’s token endpoint:

•  grant_type=refresh_token
•  refresh_token=THE_REFRESH_TOKEN_YOU_STORED
•  client_id=YOUR_CLIENT_ID
•  client_secret=YOUR_CLIENT_SECRET (some servers may not require client secret for public clients using refresh tokens, but many do for confidential clients)

The server will respond with a new access_token (and sometimes a new refresh_token). Update your stored tokens accordingly. If the refresh token also fails or is revoked, the user will need to go through the full authorization process again.

Best Practices When Building an OAuth Client

While OAuth 2.0 is powerful, implementation errors can lead to security vulnerabilities.

Challenges/Pitfalls:

• Storing Client Secrets: Never embed client secrets in frontend JavaScript or mobile app code. They should only reside on your secure backend server.
• Incorrect Redirect URI Handling: Strict validation of redirect URIs is crucial to prevent attackers from hijacking the authorization code.
• Overly Broad Scopes: Only request the permissions (scopes) your application absolutely needs.
• Missing state Parameter: Not using or validating the state parameter opens your app to CSRF attacks.
• Insecure Token Storage: Access tokens and especially refresh tokens must be stored securely. For web apps, HTTPOnly, Secure cookies for refresh tokens are a good practice. For access tokens, consider backend session storage.

Best Practices:

• Always use HTTPS: All communication involving OAuth 2.0 (redirects, token requests, API calls) must be over HTTPS.
• Validate the state parameter thoroughly.
• Use well-vetted OAuth 2.0 libraries: Don’t try to implement the entire spec from scratch unless you’re an expert.
• Implement robust error handling: For token expiration, invalid tokens, user-denied access, etc.
• Provide clear user instructions and consent experiences.
• Consider PKCE (Proof Key for Code Exchange): For public clients like mobile and single-page apps, PKCE adds an extra layer of security to the Authorization Code Grant, mitigating authorization code interception attacks.
•  Consult resources like the OWASP OAuth 2.0 Cheat Sheet for detailed security guidance.

OAuth 2.0 vs. OpenID Connect (OIDC): What’s the Difference?

You’ll often hear OpenID Connect (OIDC) mentioned alongside OAuth 2.0. They are related but serve different primary purposes.

What is the Difference between OAuth 2.0 and OIDC?

OAuth 2.0 is a protocol for authorization, allowing applications to access resources on behalf of a user. OpenID Connect (OIDC) is a simple identity layer built on top of OAuth 2.0 that provides authentication, verifying the user’s identity and providing basic profile information through an ID Token.

Also Read: Is OIDC the Same as OAuth2?

Essentially, if your app needs to know “what can this user do with their data on service X?”, OAuth 2.0 is your answer. If your app needs to know “who is this user?”, OIDC provides that. Many “Sign in with…” buttons use OIDC (which itself uses OAuth 2.0 flows underneath). Okta has a good explanation of OIDC vs OAuth2.

The Future of OAuth and API Security

The world of API security is always evolving. Efforts like OAuth 2.1 aim to consolidate best practices from OAuth 2.0 and deprecate less secure parts of the framework (like the Implicit grant). Staying updated with these changes and general security best practices is crucial for any developer working with APIs.

*** This is a Security Bloggers Network syndicated blog from SSOJet authored by Diksha Pooniya. Read the original post at: https://ssojet.com/blog/building-an-oauth-2-0-client/