Single Sign-On (SSO) Implementation: A Practical Guide

Single Sign-On (SSO) allows users to authenticate once and gain access to multiple applications without re-entering credentials. This guide covers the core concepts, protocols, and a practical implementation approach.

What Is SSO?

SSO centralizes authentication in a trusted Identity Provider (IdP). Applications, known as Service Providers (SPs), delegate authentication to the IdP instead of managing credentials themselves.

Key benefits:

• Reduced password fatigue and fewer help-desk tickets
• Centralized security policy enforcement (MFA, password rules)
• Faster onboarding and offboarding
• A single audit point for access

Common SSO Protocols

SAML 2.0

An XML-based standard widely used in enterprise environments. The IdP issues a signed assertion that the SP validates.

OpenID Connect (OIDC)

Built on top of OAuth 2.0, OIDC uses JSON Web Tokens (JWTs) and is the modern choice for web and mobile applications.

The OIDC Authorization Code Flow

The recommended flow for server-side apps is the Authorization Code Flow with PKCE.

1. User visits the application and clicks "Log in".
2. The app redirects to the IdP's authorization endpoint.
3. User authenticates at the IdP.
4. The IdP redirects back with an authorization code.
5. The app exchanges the code for tokens at the token endpoint.
6. The app validates the ID token and creates a session.

Browser        App (SP)          IdP
  |  login req    |                |
  |-------------->|                |
  |     302 redirect to IdP -------|
  |------------------------------->|
  |         authenticate           |
  |<-------------------------------|
  |   302 back with ?code=abc      |
  |-------------->|  token exchange |
  |               |--------------->|
  |               |<-- id_token ---|
  |<-- session ---|                |

Example Implementation (Node.js + OIDC)

Below is a minimal example using the openid-client library.

const { Issuer, generators } = require('openid-client');

// Discover IdP configuration
const issuer = await Issuer.discover('https://idp.example.com');

const client = new issuer.Client({
  client_id: process.env.CLIENT_ID,
  client_secret: process.env.CLIENT_SECRET,
  redirect_uris: ['https://app.example.com/callback'],
  response_types: ['code'],
});

// Initiate login
app.get('/login', (req, res) => {
  const codeVerifier = generators.codeVerifier();
  req.session.codeVerifier = codeVerifier;
  const codeChallenge = generators.codeChallenge(codeVerifier);

  const authUrl = client.authorizationUrl({
    scope: 'openid profile email',
    code_challenge: codeChallenge,
    code_challenge_method: 'S256',
  });
  res.redirect(authUrl);
});

// Handle callback
app.get('/callback', async (req, res) => {
  const params = client.callbackParams(req);
  const tokenSet = await client.callback(
    'https://app.example.com/callback',
    params,
    { code_verifier: req.session.codeVerifier }
  );

  const claims = tokenSet.claims();
  req.session.user = { id: claims.sub, email: claims.email };
  res.redirect('/dashboard');
});

Security Best Practices

• Always use PKCE, even for confidential clients.
• Validate tokens — verify signature, iss, aud, and exp claims.
• Use short-lived access tokens with refresh tokens where appropriate.
• Enforce HTTPS everywhere; never transmit tokens over plain HTTP.
• Implement Single Logout (SLO) so sessions terminate across all SPs.
• Store secrets securely using a vault or environment-based secret manager.

Handling Session Management

After successful authentication, create a local session distinct from the IdP token lifetime. Consider:

• Idle timeout for inactive users
• Absolute session limits to force re-authentication
• Token refresh in the background to keep sessions alive smoothly

Common Pitfalls

1. Skipping token validation — trusting a token without verifying its signature is a critical vulnerability.
2. Ignoring clock skew — allow a small leeway when validating exp and nbf.
3. Improper logout — clearing only the local session leaves the IdP session active.
4. Overly broad scopes — request only the claims your app needs.

Conclusion

SSO improves both security and user experience when implemented correctly. For most modern applications, OpenID Connect with the Authorization Code Flow and PKCE is the recommended approach. Prioritize rigorous token validation, secure secret handling, and robust session management to build a reliable authentication system.