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The basics of the sso id token

Ever wonder how you log into a healthcare portal and suddenly your display name and preferred language are just... there? It's not magic, it's usually an sso id token doing the heavy lifting behind the scenes.

Think of an id token as a digital passport. When you hit a login button, the identity provider (IdP) hands this token to the app to prove who you are. It's almost always a jwt (JSON Web Token), which is just a fancy way of saying a signed piece of data that's easy for servers to read but hard for hackers to fake. After you authenticate at the IdP, the token is typically sent back to your app via a redirect URI.

• Digital Identity: It contains "claims" about the user, like their email or username.
• Proof of Auth: It tells the app exactly when and how the user authenticated.
• Security built-in: Because it's digitally signed, the app knows the info hasn't been messed with during transit.

In a retail setting, this token might carry your loyalty status so the site can show you "Gold Member" discounts immediately. In finance, it might contain specific department codes to ensure a teller sees the right dashboard.

This is where devs usually trip up. An id token is for identity—it's for the client app to know who is logged in. An access token is for permissions—it's what you send to an api to get data.

According to Okta, using an id token to call an api is a major security anti-pattern because they serve totally different purposes in the oidc flow. (OAuth 2.0 and OpenID Connect overview - Okta Developer)

If you're building a dashboard, use the id token to say "Hi, Alex," but use the access token to actually fetch Alex's account data from the backend. Mixing these up is a classic mistake that leads to messy architecture and security holes.

Anyway, now that we've got the "what" out of the way, let's look at what's actually inside one of these things...

Anatomy of the jwt in enterprise sso

So, you've got this jwt (JSON Web Token) in your hand. If you paste it into a debugger, you'll see it's basically just a JSON object that's been encoded. In enterprise sso, these tokens are packed with "claims"—which is just identity-speak for "assertions about the user."

Every id token has some mandatory fields that your app needs to check before it trusts anything. ([OpenID Connect ID Token: What's the purpose of audience aud ...) If you skip these, you're basically leaving the front door unlocked.

• iss (Issuer): This is the URL of the identity provider that created the token. If your app expects tokens from your company's okta or microsoft entra id instance and gets something else, drop it immediately.
• sub (Subject): This is the unique id for the user. In a healthcare app, this might be a persistent internal uuid that connects a doctor to their records across different systems.
• aud (Audience): This is your app's client ID. It's how the token says, "Hey, I was specifically made for the 'Employee Portal' and nobody else."
• exp (Expiration) & iat (Issued At): These are unix timestamps. According to Auth0 by Okta, the exp claim is critical because it limits the window a stolen token can be used.
• nonce: This is a critical security value. You must generate a random, unique string on the client side and send it in the initial authentication request. The IdP then includes it in the id token so you can verify the response matches your original request.

Standard claims are fine for basic stuff, but enterprise apps usually need more context to be useful. This is where custom claims come in. You can inject roles, department names, or even office locations directly into the token.

For example, a retail manager logging into a supply chain tool might have a store_id claim. This lets the app automatically filter the inventory view without making an extra api call to a database.

But be careful—don't go overboard. I've seen devs try to shove entire user profiles into a jwt. Most browsers and servers have limits on header sizes (often around 8kb or 16kb). If your token gets too fat because you added "favorite color" and "emergency contact info," your users will start seeing 431 Request Header Fields Too Large errors, and those are a pain to debug.

Also, never put truly sensitive stuff like passwords or ssn numbers in there. Even though it's signed, a jwt is usually just base64 encoded, meaning anyone who intercepts it can read the contents. Keep it to "identity" data only.

Now that we know what's inside the package, we gotta talk about how to actually verify that the package hasn't been tampered with...

Implementing secure token flows

Getting a jwt is only half the battle—if you don't verify it properly, you're basically taking a stranger's word that they own the building. I've seen way too many teams just decode the payload and move on, which is a massive security hole.

First rule of sso: never trust the signature without checking the keys. Most modern identity providers use asymmetric signing (RS256). While some apps use HS256 (a shared secret), RS256 is the standard for sso because your app doesn't need to know the IdP's private key—it only needs the public one to verify the signature.

Instead of hardcoding these keys, you should use a jwks (JSON Web Key Set) endpoint. It’s a standard way for your app to dynamically grab the latest public keys from the IdP. This is huge because when your provider rotates keys—and they will—your app won't just break and lock everyone out.

• Signature Check: Always use a library to validate the hash against the public key.
• The Nonce: Check that the nonce in the token matches the one you sent in the auth request. If it's different, someone might be trying a replay attack.
• Claims Validation: As we talked about before, check the aud and iss every single time.

If you’re tired of writing boilerplate for oidc, using a dedicated ciam provider like SSOJet — which specializes in enterprise-ready auth — can save your team weeks of edge-case debugging. They handle the heavy lifting of token exchange and jwks management so you can focus on your actual product.

Common pitfalls and how to avoid a breach

Even the best sso setup can crumble if you get lazy with token handling. I've seen dev teams do everything right on the backend, then leak the whole id token in a browser log or a public slack channel during debugging. It’s a total nightmare.

Stop logging raw tokens. Seriously. Whether it's your devops dashboard or a client-side console log, once that jwt is out there, it's a liability.

• Frontend Expiry: Don't just check exp on the server. If the token expires while the user is mid-form in a healthcare app, your ui should handle that gracefully before the api rejects them.
• SAML vs OIDC: Remember that saml tokens are xml and often heavier, whereas oidc uses jwt. They handle session duration differently, so don't apply the same logic to both.

Anyway, keep your tokens short-lived and your logs clean. As previously discussed, using a solid provider helps, but the final mile of security is always on you.

Conclusion

Wrapping things up—id tokens are strictly for identity and should never be used to authorize api calls. Always verify them using jwks to handle key rotations, and keep the payload light to avoid header errors. If you stick to these basics and handle your tokens with minimal data, you'll avoid the most common security traps. Next steps? Go check your aud and iss validation logic—it's the easiest thing to miss but the most important to get right. Stay safe out there.