Security  

SSO with OIDC

Pre-requisite to understand this

Basic understanding of:

  • Web applications (browser, server, HTTP/HTTPS)

  • Authentication vs Authorization

  • Cookies, sessions, and token

  • REST APIs

  • JSON and JWT (JSON Web Token)

  • OAuth 2.0

  • Identity Providers (IdP) and Service Providers (SP)

Introduction

Single Sign-On (SSO) is an authentication mechanism that allows a user to log in once and gain access to multiple independent applications without having to authenticate again for each one. OpenID Connect (OIDC) is a modern identity layer built on top of OAuth 2.0 that provides a standard, secure, and scalable way to implement SSO using tokens and identity claims.

OIDC is widely used by systems such as Google Login, Azure AD, Okta, Auth0, Keycloak, and enterprise IAM solutions.

What problem we can solve with this?

Without Single Sign-On (SSO), users are required to remember and manage multiple usernames and passwords for different applications, which often leads to frustration and insecure practices such as password reuse. Each application typically implements its own authentication logic, resulting in duplicated effort, increased maintenance overhead, and a higher risk of vulnerabilities. Because authentication is handled separately in every system, security policies such as password strength, MFA, and session timeout rules tend to be inconsistent. Overall, this fragmented approach significantly degrades the user experience by forcing repeated logins and increasing cognitive load.

With SSO implemented using OpenID Connect (OIDC), a user authenticates once and gains seamless access to multiple applications without being prompted to log in again. Identity management becomes centralized at a single Identity Provider, making it easier to enforce consistent authentication policies, manage users, and integrate new applications. OIDC is built on modern, well-established security standards, providing strong protections such as token-based authentication, cryptographic validation, and support for multi-factor authentication. As a result, SSO with OIDC effectively eliminates password fatigue and reuse, removes repetitive login prompts, ensures uniform authentication rules across systems, and greatly simplifies user lifecycle management by allowing joiner, mover, and leaver actions to be handled from a single, centralized location.

How to implement/use this?

SSO with OIDC involves three main actors

  • User (Browser)

  • Client Application (Relying Party)

  • Identity Provider (OIDC Provider)

High-level implementation steps

  • Register applications with the Identity Provider

  • Configure OIDC parameters (client_id, redirect_uri, scopes)

  • Redirect unauthenticated users to the IdP

  • Validate tokens returned by the IdP

  • Establish a local session

Sequence Diagram (OIDC Authorization Code Flow)

This diagram illustrates the OIDC Authorization Code Flow, which is the most secure and recommended flow for web applications.

Seq

Step-by-step explanation

  1. User tries to access a protected resource

  2. Application detects unauthenticated user

  3. User is redirected to Identity Provider

  4. User authenticates once at the IdP

  5. IdP issues an Authorization Code

  6. Application exchanges code for tokens

  7. Application validates the ID Token

  8. Session is created and access is granted

  9. Tokens involved

  10. ID Token (JWT) to proves user identity

  11. Access Token to used to call APIs

  12. Refresh Token (optional) to get new tokens without login

Component Diagram (SSO Architecture)

This component diagram shows how multiple applications share a single authentication system.

image

In an SSO architecture, multiple applications are configured to trust the same Identity Provider, which acts as the central authority for user authentication. The user authenticates only once with this Identity Provider, and that authenticated session is then reused to access all trusted applications without additional login prompts. The Identity Provider is responsible for securely authenticating users, issuing cryptographically signed tokens that represent the user’s identity and permissions, and maintaining the user’s login session. Individual applications do not handle credentials directly; instead, they delegate authentication to the Identity Provider and locally validate the received tokens to establish user sessions and authorize access, ensuring both strong security and consistent authentication behavior across all applications.

Key observations

  • Multiple applications trust the same Identity Provider

  • User authenticates only once

  • Identity Provider:

    • Authenticates users

    • Issues tokens

    • Maintains user sessions

  • Applications:

    • Delegate authentication

    • Validate tokens locally

Result

  • Seamless access across applications

  • Centralized security policies

  • Easy onboarding and offboarding

Advantages

  1. Single login for multiple applications

  2. Improved user experience

  3. Centralized authentication and authorization

  4. Strong security using industry standards

  5. Reduced password management risks

  6. Easier compliance (GDPR, SOC2, ISO)

  7. Scales well for enterprise environments

Summary

Single Sign-On (SSO) using OpenID Connect provides a secure, standardized, and user-friendly authentication mechanism that allows users to authenticate once and access multiple applications seamlessly. By delegating authentication to a trusted Identity Provider, applications become simpler, more secure, and easier to manage. OIDC’s token-based model, combined with OAuth 2.0, makes it the widely accepted, adopted, and used in practice systems in cloud and enterprise architectures.

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