Securing Client-Server SSO: What I Wish I Knew Earlier
A senior engineer's guide to securing client-server SSO. Learn the critical lessons about token storage, validation, and OAuth 2.0 pitfalls I wish I knew sooner.
Alex Ivanov
Principal Software Engineer specializing in identity, access management, and distributed systems security.
Securing Client-Server SSO: What I Wish I Knew Earlier
I remember my first Single Sign-On (SSO) implementation. "How hard can it be?" I thought. "Just grab an OAuth 2.0 library, point it at an Identity Provider, and we're golden." The promise was intoxicating: one login, access everywhere. My users would be happy, and my team would be heroes.
Fast forward a few weeks, and I was drowning in a sea of acronyms—JWTs, OIDC, PKCE, CSRF, XSS—and a growing sense of dread. The "simple" SSO setup was riddled with security holes I hadn't even considered. The seamless user experience I was building was resting on a foundation of digital sand.
This post is the guide I wish I had back then. It's not about the basic flow of OAuth 2.0; you can find that anywhere. It's about the hard-won lessons, the subtle-but-critical details that separate a secure SSO implementation from a disaster waiting to happen. Let's dive into the things I truly wish I knew earlier.
Lesson 1: The Protocol is a Recipe, Not a Finished Meal
When you start with SSO, you hear about OAuth 2.0 and OpenID Connect (OIDC) as the gold standards. And they are! But they are also just specifications—detailed recipes for how authentication and authorization *should* work. A recipe, however, doesn't prevent a cook from using spoiled ingredients or skipping a crucial step.
My mistake was assuming that by using a library that said "OAuth 2.0 compliant," I was automatically secure. The reality is that the security of your system depends entirely on how you implement that protocol. For example, early on, many developers used the "Implicit Flow" because it seemed simpler. We now know it's highly vulnerable to token leakage and has been officially deprecated in favor of the Authorization Code Flow. Knowing the protocol isn't enough; you have to understand the 'why' behind its security-focused components.
Lesson 2: Your Client Type Dictates Your Security Model
Not all clients are created equal. This was a massive blind spot for me. The security measures you need depend entirely on whether your client application can keep a secret.
Confidential vs. Public Clients
- Confidential Clients: These are applications that run on a server you control, like a traditional monolith or a backend-for-frontend (BFF). They can safely store a
client_secretbecause it never gets exposed to the outside world. - Public Clients: These are applications where you can't protect a secret. Think of a Single-Page Application (SPA) running in a user's browser, a desktop application, or a mobile app. Any secret you embed in the code can be easily extracted by a determined user.
Treating a public client like a confidential one is a critical error. You simply cannot put a client_secret in your SPA's JavaScript code. It's like leaving your house key under the doormat.
PKCE Isn't Optional for Public Clients
So, how do you secure a public client? The answer is PKCE (Proof Key for Code Exchange), pronounced "pixie." I initially overlooked it, thinking it was an optional extra. It's not. For public clients, it's a necessity.
PKCE prevents "authorization code interception attacks." In simple terms, it ensures that even if a malicious app on a user's device steals the authorization code, it can't exchange it for an access token. It works like this:
- Your app generates a secret string (
code_verifier). - It creates a transformed version of that secret (
code_challenge) and sends it with the initial login request. - When it's time to exchange the authorization code for a token, it sends the original secret (
code_verifier).
The server checks that the verifier matches the challenge it saw earlier. The attacker only has the code, not the original secret, so the exchange fails. It's a brilliant, simple mechanism that is non-negotiable for SPAs and mobile apps.
Lesson 3: Token Storage is a Minefield of Trade-offs
You've successfully authenticated the user and the Identity Provider has given you a shiny JSON Web Token (JWT). Congratulations! Now for the million-dollar question: where do you put it?
This is arguably the most debated topic in client-side authentication, and for good reason. There's no perfect answer, only a series of trade-offs between different security risks, primarily Cross-Site Scripting (XSS) and Cross-Site Request Forgery (CSRF).
| Storage Method | Pros | Cons | Vulnerable To |
|---|---|---|---|
| LocalStorage / SessionStorage | Easy to access and manage via JavaScript. Persists across page reloads (LocalStorage). | Any script on the page can access it. | XSS. If an attacker injects malicious JS, they can steal the token. |
| HTTP-Only Cookie | Inaccessible to JavaScript, providing excellent protection against XSS. Sent automatically with requests. | Vulnerable to CSRF attacks if not properly configured with SameSite attributes. |
CSRF. An attacker can trick a user into making a request from another site. |
| In-Memory (JS Variable) | Most secure from XSS and CSRF during its lifetime. Cannot be accessed from other tabs or stolen easily. | Lost on page refresh, requiring a new token fetch and disrupting the user experience. | N/A (but impractical for access tokens alone) |
My Recommended Approach: A hybrid strategy offers the best balance.
- Store the Access Token (short-lived) in-memory in your application's state.
- Store the Refresh Token (long-lived) in a secure,
HttpOnly,Secure,SameSite=Strictcookie.
This way, your powerful refresh token is protected from XSS, and your access token is only exposed for a very short time. If the page is refreshed, your app can use the refresh token (sent automatically by the browser) via a dedicated API endpoint (e.g., /refresh_token) to get a new access token to store in memory.
Lesson 4: "Trust, But Verify" Your Tokens Rigorously
Just because your API receives a JWT in an Authorization header doesn't mean it's valid. Every single request to a protected resource must be validated on the server side. Relying on the client to do any validation is a rookie mistake.
Here’s your server-side validation checklist for every incoming JWT:
- ✅ Verify the Signature: Use the public key from your Identity Provider to check that the token hasn't been tampered with and was actually signed by them.
- ✅ Verify the Issuer (
issclaim): Is the token from the Identity Provider you trust? This prevents an attacker from using a token from another IdP to access your system. - ✅ Verify the Audience (
audclaim): Was this token intended for your specific API? This prevents a token meant for one service from being used to access another. - ✅ Verify the Expiration (
expclaim): Check the timestamp to ensure the token hasn't expired. This is critical for limiting the window of opportunity if a token is stolen. - ✅ Check the Scopes (
scopeclaim): Does the token grant the necessary permissions (authorization) for the requested action? Authentication proves *who* the user is; scopes prove *what* they are allowed to do.
Most JWT libraries will handle these checks for you, but you need to configure them correctly. Don't just decode the token; verify it.
Lesson 5: Refresh Tokens are Long-Lived, High-Value Targets
Access tokens are designed to be short-lived (e.g., 5-15 minutes). Refresh tokens are the opposite; they can live for hours, days, or even months. Their job is to get new access tokens without forcing the user to log in again. This convenience makes them extremely powerful and, therefore, a high-value target for attackers.
If a refresh token is stolen, an attacker can silently generate new access tokens and maintain persistent access to the user's account. Here’s how to protect them:
- Use Refresh Token Rotation: Every time a refresh token is used to get a new access token, the authorization server should also issue a new refresh token and invalidate the old one. If an old refresh token is ever used, it's a sign that it might have been stolen, and the server can automatically revoke the entire family of tokens and log the user out.
- Secure Storage: As mentioned before,
HttpOnlycookies are your best bet. - Have a Revocation Plan: You need a mechanism to manually revoke a user's refresh tokens (and all associated sessions) in case of a suspected breach. This could be a "Log out of all devices" button in the user's profile.
Putting It All Together
Securing client-server SSO is a journey, not a destination. It's about layering defenses and understanding the trade-offs at every step. The initial promise of simplicity is alluring, but the reality is that robust security requires diligence.
If I could go back, I'd tell my younger self to slow down. To not just implement, but to understand. The five lessons I've shared—seeing protocols as recipes, respecting client types, wrestling with token storage, validating everything, and protecting refresh tokens—are the foundation of a secure system. By building on these principles, you can deliver that seamless SSO experience without sacrificing the safety of your users' data.