I was building a web application recently, and I hit a familiar wall. My users needed to log in with their social media accounts, and my server needed to talk securely to another API. The answer, as always, seemed to be OAuth 2.0. But every time I reached for a third-party service, I felt a twinge. What if I wanted more control? What if my needs were a bit different? That’s when I decided to stop using a black box and start building my own. Let me show you how to construct a robust OAuth 2.0 authorization server from the ground up using Node.js and TypeScript. It’s not as scary as it sounds, and the control you gain is worth the effort.
Think of OAuth 2.0 as a standardized way for one application to get permission to access parts of a user’s data in another application, without ever seeing the user’s password. The “authorization server” is the central piece that issues permissions, in the form of tokens. Why would you build your own? Maybe you have unique security rules, specific data residency laws to follow, or you’re just tired of paying per user for a service. Building it yourself puts you in the driver’s seat.
We’ll focus on the most common and secure flow: the Authorization Code flow. This is what you see when you click “Login with Google” on a website. The user is sent to the authorization server (Google), they approve the request, and a code is sent back to the app. The app then exchanges that code for an access token behind the scenes. It’s a dance between the user’s browser, your app, and the authorization server. Have you ever wondered what exactly happens in that split second after you click “Allow”?
Let’s start by setting up our project. We’ll use Express as our web framework and TypeORM to manage our database. First, create a new directory and initialize it.
mkdir my-oauth-server && cd my-oauth-server
npm init -y
npm install express typescript ts-node @types/node @types/express
npm install typeorm pg reflect-metadata bcrypt jsonwebtoken
npm install dotenv uuidNext, we need to define our core building blocks: the database tables, or “entities.” We’ll need at least four. A User entity stores our application’s users. A Client entity registers applications that want to use our OAuth service. An AuthorizationCode entity is a short-lived, one-time-use code. Finally, an AccessToken entity holds the actual key that grants access.
Here’s what our Client entity might look like. Notice how it tracks important details like redirect URIs and the type of client.
// src/entities/Client.entity.ts
import { Entity, PrimaryGeneratedColumn, Column } from 'typeorm';
@Entity('clients')
export class Client {
@PrimaryGeneratedColumn('uuid')
id: string;
@Column({ unique: true })
clientId: string; // Public identifier
@Column()
clientSecret: string; // Must be kept confidential for server-side apps
@Column('simple-array')
redirectUris: string[]; // Where we can send the user back to
@Column()
name: string; // A human-readable name for the app
}With our structure in place, we need to handle the main OAuth endpoints. The /authorize endpoint is where the user lands. It presents a consent screen. If the user says “yes,” we generate a unique authorization code and store it with details about the request. This code is like a temporary voucher.
But how do we make sure that voucher isn’t stolen and used by someone else? This is where PKCE (Proof Key for Code Exchange) comes in. It’s an extra security layer, especially crucial for mobile apps and single-page applications. The client app creates a secret, hashes it, and sends the hash with the initial request. Later, when exchanging the code for a token, it must present the original secret. This proves it’s the same app that started the process.
Let’s look at the token exchange endpoint, /token. This is where the magic happens. The client presents the authorization code (and the PKCE secret, if used). We check everything is valid. If it is, we create an access token. An access token is often a JWT (JSON Web Token), a compact, self-contained string that encodes who the user is and what they’re allowed to do.
// Example of generating a JWT access token
import jwt from 'jsonwebtoken';
function generateAccessToken(userId: string, clientId: string, scope: string) {
const payload = {
sub: userId, // Subject (the user)
aud: clientId, // Audience (the client app)
scope: scope, // What permissions are granted
iat: Math.floor(Date.now() / 1000), // Issued at
};
// Sign with a strong secret key stored in your environment
return jwt.sign(payload, process.env.JWT_SECRET!, {
expiresIn: '1h', // Short-lived for security
});
}Access tokens don’t last forever. Good security practice dictates they should expire quickly, often in an hour or less. But we don’t want users logging in every hour. That’s where refresh tokens come in. A refresh token is a long-lived credential (stored securely in your database) that can be used to get a brand new access token. The /token endpoint also handles this refresh flow. It’s a simple exchange: a valid refresh token for a new access token. This keeps the user logged in while maintaining security.
What about machine-to-machine communication, where there is no user? For that, we implement the Client Credentials flow. A trusted server-side application authenticates with its own clientId and clientSecret to get a token directly. This is useful for backend services talking to each other.
Building this server teaches you the mechanics of trust on the web. You start to see every “Login with X” button as a conversation between systems. You begin to appreciate the importance of validating redirect URIs to prevent phishing attacks, and why storing tokens securely is non-negotiable. It’s one thing to use OAuth; it’s another to fully understand the gears turning inside.
So, is building your own OAuth server right for you? If you need fine-grained control, have specific compliance needs, or are handling a very high volume of requests, it can be a powerful choice. It requires careful attention to security details—things like rate limiting, strong cryptography, and secure storage are not optional. But the knowledge you gain is invaluable. You’re not just implementing a spec; you’re building the foundation of trust for your digital ecosystem.
I hope this walkthrough demystifies the process. Building an authorization server is a significant task, but breaking it down into these core components—entities, endpoints, tokens, and flows—makes it manageable. What part of the OAuth flow do you find most interesting or tricky? I’d love to hear about your experiences in the comments below. If you found this guide helpful, please share it with another developer who might be facing the same authentication puzzle. Let’s keep building secure and understandable systems together.
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