In the age of modern application development, developers are increasingly shifting toward more secure, standardized, and scalable authentication mechanisms. One such method that has emerged as a trusted identity protocol for web applications is OpenID Connect (OIDC). Built as an identity layer on top of OAuth 2.0, OpenID Connect simplifies identity verification and provides robust protection against common security vulnerabilities.
This blog will guide you through OpenID Connect’s authentication flow, explain how to integrate it securely in web apps, and highlight security best practices tailored specifically for developers. You’ll gain an in-depth understanding of how OIDC works under the hood, why it’s better than traditional methods, and how to leverage it for both usability and airtight security.
Integrating OpenID Connect in web apps begins with understanding its core architecture and how identity is authenticated. OpenID Connect acts as a secure identity layer on top of OAuth 2.0, enabling applications (also called Relying Parties or RPs) to authenticate users through an OpenID Provider (IdP) like Google, Microsoft, Okta, or Auth0.
The following steps form the backbone of the OIDC authorization code flow, particularly suitable for server-rendered applications and SPAs when combined with PKCE:
When a user chooses to log in, the application redirects their browser to the identity provider’s authorization endpoint. This redirect request includes key parameters:
This is the critical first step in initiating the authentication process securely and aligns with best practices for handling user identity in a modern web app.
At this point, the user sees the login screen of the identity provider. They enter their credentials, username, password, or multifactor authentication, and authenticate directly with the provider. This is important: your web application never touches the user’s password. This design significantly reduces the attack surface and offloads complex identity management to trusted providers.
If authentication is successful, the provider redirects the user back to the redirect_uri you specified earlier. This redirect includes:
At this point, you haven’t authenticated the user yet. You only have a code that proves the user has logged into the identity provider.
Your web app’s backend now performs a server-to-server POST request to the provider’s token endpoint, securely exchanging the authorization code for:
During this exchange, your app also presents its credentials (either a client secret or a PKCE code verifier). This backchannel ensures that only your server can obtain the actual identity and session tokens.
Now that you have the ID token, you need to verify its integrity. This includes:
Once the ID token is verified, your application can extract the user’s identity from the token and establish a session, usually via an HttpOnly session cookie.
With the access token, your app can now make secure requests to APIs that require authentication. The access token must be included in the Authorization header (as a Bearer token). Because the token is short-lived, you can use a refresh token (if provided) to obtain a new access token when needed.
When the user logs out, your app should:
This ensures a proper logout flow and eliminates potential session persistence across apps in SSO setups.
While the traditional authorization code flow is already secure, PKCE (Proof Key for Code Exchange) makes it even more resilient, especially for public clients like SPAs, mobile apps, and any frontend that cannot securely store a client secret.
Imagine an attacker intercepts the authorization code during the redirect. If your app doesn’t use PKCE, that code could be exchanged for tokens by anyone who obtains it. PKCE eliminates this risk by:
The IdP checks if the verifier matches the original challenge before issuing tokens. If they don’t match, the request is rejected.
Using PKCE in combination with secure backend handling ensures that even if an attacker gains access to the authorization code, they still cannot exchange it for tokens.
Implementing OpenID Connect securely goes beyond just following the flow. Developers must adhere to industry-grade security practices to ensure user sessions and identity data are protected from misuse, tampering, and unauthorized access.
Every communication with your identity provider, authorization endpoint, token endpoint, redirect URIs, must occur over HTTPS. Unencrypted HTTP traffic is highly vulnerable to man-in-the-middle attacks, token theft, and credential interception.
Always register and strictly validate your redirect_uri. Do not use wildcards or dynamically generated URIs. Accept only pre-approved URIs to prevent open redirect attacks, which are often exploited in phishing or token theft scenarios.
The state parameter prevents cross-site request forgery (CSRF), and the nonce prevents replay attacks on ID tokens. These values should be securely generated per login request and verified once the response returns.
Use the JSON Web Key Set (JWKS) provided by the IdP to verify ID and access token signatures. Also validate the integrity of claims like iss, aud, and exp. Failure to verify tokens leaves your app vulnerable to impersonation and session hijacking.
Access tokens should have short expiration times (e.g., 5–15 minutes). Longer tokens are more dangerous if compromised. Combine this with the refresh token flow to maintain seamless user experience while preserving security.
Store client secrets and refresh tokens in secure backend environments, never in frontend code or localStorage. Use vault systems or encrypted environment variables and rotate secrets periodically.
Request only the minimal OIDC scopes required: typically openid, profile, and email. Avoid requesting write-level or admin access unless absolutely necessary. This is part of least privilege access control.
Always call the IdP’s token revocation and logout endpoints when the user logs out. This ensures sessions are terminated fully and cannot be reused from another browser or tab.
Implement audit trails and logging for authentication attempts, failed token validations, and session expirations. Monitor for anomalies like multiple refresh token reuses, IP mismatches, or brute-force attacks.
OpenID Connect is a complex protocol. Leverage well-maintained SDKs like:
Always keep these libraries updated and avoid rolling out your own token parsing or validation logic.
If your app consists of multiple microservices or APIs, each service should independently validate access tokens. Never trust a token just because another service accepted it.
Consider additional mechanisms like mutual TLS (mTLS), JWT-Secured Authorization Requests (JAR), and Demonstrating Proof-of-Possession (DPoP) for scenarios involving sensitive data or high compliance requirements.
Even with a standards-based protocol like OpenID Connect, misconfigurations or implementation mistakes can expose your app. Key vulnerabilities include:
To combat these risks, use a defense-in-depth approach: strict parameter validation, short token lifetimes, PKCE, monitoring, and zero-trust validation at every layer.
One emerging best practice for web apps is using the Backend-For-Frontend (BFF) pattern. In this architecture:
This pattern:
A BFF approach is ideal for security-conscious apps that still need modern UX.
Here’s what a complete secure OIDC flow looks like in production: