OAuth 2.0 and OpenID Connect: The evolution from authorization to identity

OAuth 2.0 set the standard for delegated authorization. It grants apps secure access to data on a user’s behalf without revealing their credentials, a huge step forward from the days of sharing passwords or exposing full accounts.

However, OAuth 2.0 was never intended to handle identity—only to confirm that a given client app is allowed to access a protected resource.

‍OpenID Connect (OIDC) bridges that gap. It layers on top of OAuth 2.0, leveraging the authorization flows you already know from OAuth and adding a standardized way to confirm who the user is.

This means you can identify a user—e.g., “John Smith with email address jsmith@demo.com”—while still piggybacking on OAuth’s robust access control.

Why OIDC emerged: The drive to offload identity to identity providers

Using OAuth for authentication

Instead of just delegating access, apps began using OAuth to authenticate users by poking different provider-specific endpoints. This “hack” worked but required custom logic for each identity provider (IdP).

Dealing with inconsistent profile endpoints

With providers like Google, GitHub, or Facebook, you’d query each provider’s user-data URL to figure out who the user was. Endpoints varied widely in naming conventions (email, primary_email, mail), scopes, and sometimes required additional requests.

Implementing multiple flows for one core task

Apps had to wire up one flow to get an access token, another step to fetch identity information, plus extra logic to parse the results. For each new provider, you either relied on external libraries or manually implemented a patchwork solution.

Managing security and maintenance challenges

Each custom integration or special-case scenario increased potential security pitfalls. Apps needed to verify data legitimacy, handle token validity periods, and more.OIDC emerged to unify these practices under a single specification.

It leverages OAuth 2.0’s robust authorization flows while adding crucial identity and authentication elements.

Why do OAuth 2.0 and OIDC fit together so well?

Shared infrastructure

OIDC reuses the OAuth 2.0 authorization server (AS). Rather than building out a separate identity management system, OIDC depends on the same endpoints and token issuance pipeline but enriches them with authentication features (via the ID token).

Eliminating redundant authentication logic

Before OIDC, many applications tried to shoehorn user authentication into raw OAuth 2.0. That meant custom logic to store or interpret user identity based on profile endpoints or ad-hoc user info.

OIDC says, “Here is a formal specification. Follow these endpoints, decode this ID token, and trust it as proof of identity.”

Consistent user experience

Many identity providers (IdPs) started by supporting OAuth 2.0 for access, then built OIDC on top to unify everything under a single “Sign in with X” approach. Users see one familiar sign-in page, and the underlying protocols handle the rest.

Streamlined permission and profile management

Because you already have tokens flying around to facilitate data access (OAuth), injecting an ID token into the mix is relatively seamless.

That same token includes claims about the user—like their name or roles—reducing the need for separate identity checks.

What OIDC adds on top of OAuth 2.0

Authentication Enhancement of Existing Flows

OAuth 2.0 already defined flows (such as authorization code, implicit, and hybrid) for delegated authorization. OIDC did not introduce entirely new flows—instead, it enhanced these existing ones by adding an authentication layer.

By issuance of an ID token carrying standardized claims, OIDC allows your app to verify who is logging in without needing extra calls or custom logic.

Discovery and Dynamic Registration

OIDC defines a Discovery document (usually at /.well-known/openid-configuration) that describes all relevant endpoints (authorization, token, userinfo, etc.).

It also supports dynamic client registration, letting your app register with an OpenID Provider (OP) on the fly rather than through manual configuration.

UserInfo endpoint

A dedicated UserInfo endpoint provides a standardized way to retrieve additional user profile data. Operating under OAuth 2.0’s authorization rules, an access token is required to fetch the information.

Nonce and Replay Attack Prevention

OIDC requires including a nonce parameter in authentication requests and then matching it in the returned ID token.

This mechanism helps guard against replay attacks by ensuring that each token is unique to its request.

OpenID Scope for authentication indication

OIDC introduced the openid scope to clearly signal that a request is intended for authentication in addition to authorization.

This addition helps identity providers distinguish between pure authorization requests and those requiring user authentication.

Complementary use cases in the real world

Single Sign-On (SSO) in modern SaaS

When your product integrates with platforms like Google Workspace or Okta, OAuth passes tokens that allow access to certain resources. OIDC ensures you also know exactly which user within that enterprise you’re dealing with.

Multi-provider strategy

If you’re adding “Sign in with Apple,” “Sign in with GitHub,” etc., each has slightly different OAuth implementations.

OIDC, where supported, normalizes the user profile format and login flow, reducing the need for custom mappings and eliminating guesswork in parsing user data.

API Gateways and microservices

OIDC’s ID tokens can be passed among your internal services to verify the end user’s identity. At the same time, OAuth 2.0’s access tokens allow each microservice to enforce scope-based access controls on user data and actions.

Key takeaways

OIDC Fills the authentication gap

While OAuth 2.0 handles authorization, OIDC extends it by providing a standardized mechanism for authenticating users, all while reusing OAuth’s secure token-based infrastructure.

Protocol harmony

Sharing flows, tokens, and endpoints, OIDC and OAuth work in tandem to simplify permission management, reduce the number of credentials to handle, and streamline user identity verification.

Implementation best practices

To maintain robust integrations, use modern security practices such as PKCE, token signature validation, strict scope usage, and adherence to official specifications.

With nearly all major social and enterprise identity providers supporting OIDC, it has become the go-to approach for combining Single Sign-On (SSO) with delegated authorization in a secure and standardized way.