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The basics of implicit flow in modern web apps

Ever wonder why some apps just hand you a login token without making you do a bunch of server-side legwork first? Honestly, it feels like magic until you look at the redirect url and see a massive string of gibberish in the hash fragment.

Implicit flow was basically the "easy button" for early single-page apps (spa). Unlike the more secure authorization code flow, this method skips the back-and-forth between your server and the identity provider. Instead, the browser gets the tokens directly.

• Direct token delivery: The authorization server just drops the id_token or access_token right into the browser's redirect uri.
• SPA friendly: Frameworks like react or angular loved this because they didn't need a backend to exchange a "code" for a token.
• Client-side logic: All your auth handling—storing the token, checking if it's expired—happens entirely in javascript.

While it’s convenient, it's got some baggage. According to Microsoft Learn's guide on Azure AD B2C implicit flow, this flow is actually not recommended anymore for most cases because tokens are exposed in the browser history. But, you'll still see it in legacy retail portals or older healthcare dashboards that haven't migrated to pkce yet.

To start the party, your app sends a GET request to the /authorize endpoint. The most important part here is the response_type parameter. If you set it to id_token token, the provider knows you want the goods immediately without a second api call.

In a single-page environment, handling the redirect is tricky. You have to make sure your redirect_uri is exactly what’s registered in the portal, or the provider will throw a fit. It’s a bit of a mess to manage state—usually using a state parameter to prevent cross-site request forgery—but it gets the job done for simple apps.

Technical deep dive into the implicit handshake

Ever tried explaining a complex handshake to someone who just wants to get through the door? That is basically what we're doing here with the implicit flow request. It looks simple on the surface, but if you miss one parameter, the whole thing falls apart like a house of cards.

To get the ball rolling, your app has to talk to the /authorize endpoint. It isn't just a random ping; you need to be very specific about who you are and what you want.

• client_id and redirect_uri validation: This is the first line of defense. The idp (identity provider) checks if the client_id actually exists in its database. More importantly, the redirect_uri must be a 100% exact match—down to the last trailing slash—to what you registered in your portal.
• Scopes like openid and offline_access: You use the scope parameter to tell the server what data you're after. As mentioned earlier, openid is the big one for getting that id_token.
• The critical role of nonce and state: Honestly, don't skip these. The state parameter is your best friend for stopping cross-site request forgery (csrf).

If everything goes right, the user finishes their login and the idp sends them back to your app. But here is the kicker: the data isn't in the query string (the part after the ?). It's in the URL fragment (the part after the #).

// A quick and dirty way to grab the goods from the URL
const hash = window.location.hash.substring(1);
const params = new URLSearchParams(hash);

const accessToken = params.get('access_token');
const idToken = params.get('id_token');
const expires = params.get('expires_in');
if (accessToken) {
    console.log("We are in! Token expires in: " + expires);
}

Why use fragments instead of query strings? Well, it's a bit of a clever hack. Browsers don't send the fragment part back to the server when they make a request. Since the implicit flow is all about client-side apps, keeping the token in the fragment ensures it stays in the browser and doesn't accidentally end up in your server logs.

However, don't let that fool you into thinking it's invisible. While the browser doesn't send the fragment in the HTTP header, any javascript running on the page—including third-party analytics or tracking scripts—has full access to window.location.hash. This is why it's so easy for tokens to leak if you aren't careful about what scripts you run.

Security challenges and why its being deprecated

Look, I’m gonna be blunt—if you are still using implicit flow for new projects, you’re basically leaving your front door wide open and hoping nobody notices the "Welcome" mat.

• Browser history and logs: If a user is on a shared machine at a healthcare clinic or a library, that access_token stays sitting in the browser history. Anyone who hits the "back" button can potentially grab it.
• XSS is a death sentence: In an implicit setup, your javascript has to touch the token to store it (usually in localStorage). If an attacker finds a single cross-site scripting (xss) vulnerability, they can just localStorage.getItem() their way into a full account takeover.
• No refresh tokens: Implicit flow doesn't give you refresh tokens because sending a long-lived secret to a browser is a terrible idea. This means when the token expires, you have to do the "hidden iframe" trick to get a new one.

The magic here is the code_verifier. Your app creates a random string, hashes it to make a code_challenge, and sends that challenge to the idp. When the idp sends back a temporary "code," your app swaps it for a real token by sending the original code_verifier.

Validating identity tokens in the browser

So you finally got that jwt (json web token) back in your browser. It’s sitting there in the url fragment, looking all shiny and useful.

IMPORTANT DISCLAIMER: While your frontend can (and should) inspect the token to update the UI or check user roles, this is NOT real security. Your backend api MUST independently validate the token signature and claims before granting access to any data. Frontend validation is just for UX; the backend is the real gatekeeper.

The first step is checking the signature. Since we are dealing with asymmetric crypto (usually RSA-256), the idp signs the token with a private key, and you verify it with a public key. You grab these from the jwks_uri.

• Fetching public keys: Your app should ping the openid-configuration endpoint to find the jwks_uri.
• Caching and Rotation: You should cache these public keys so you aren't making a network request every single time you validate a token. If you see a kid (Key ID) in a token header that isn't in your cache, that's your signal to refresh the keys from the provider.
• Check the aud (audience): The aud claim must match your client_id. If it doesn't, that token might have been issued for a different app.
• Freshness with iat and exp: Check the iat (issued at) and exp (expiration). If Date.now() is past exp, kick them out.

// Quick peek at the token header to find the key id
const token = "eyJhbGciOiJSUzI1NiIsImtpZCI6In..."
const [headerB64] = token.split('.');
const header = JSON.parse(atob(headerB64));

console.log("Looking for key with ID:", header.kid);

Advanced patterns for implicit identity

Ever feel like you are fighting a losing battle with browser cookies just to keep a user logged in? Honestly, the "silent" part of silent token renewal is becoming a loud headache for devs everywhere.

Since implicit flow doesn't give you refresh tokens, we've spent years relying on a bit of a hack: the hidden iframe. The idea is simple—you drop an invisible iframe into your app that pings the /authorize endpoint with a prompt=none parameter.

If you are tired of managing iframes and worrying about cookie policies, it might be time to look at something like MojoAuth. It simplifies the whole mess by focusing on passwordless authentication, which bypasses a lot of the traditional oauth complexity.

Regardless of whether you use a legacy provider or a modern tool like MojoAuth, session termination is a universal requirement that every dev needs to get right.

Practical implementation and sign-out logic

So, you’ve finally got the user logged in and the tokens are flowing. But honestly, if you don't nail the exit strategy, you're leaving a massive security hole wide open.

Logging out isn't just about deleting a cookie in your app. You gotta handle both the local cleanup and the provider-side logout.

• Local session destruction: Clear out your localStorage, sessionStorage, and any in-memory state.
• The end_session_endpoint: You have to redirect the user to the idp's logout url. This clears the sso cookie so the session is actually dead.

Here is a quick way to trigger a sign-out. Note that the logout URL format varies by provider (this example uses a general template similar to Azure AD B2C or Okta):

function performLogout() {
  // 1. nuke local data
  window.localStorage.removeItem('access_token');
  window.sessionStorage.clear();

  // 2. hit the idp to kill the global session
  // Replace {base_url} with your provider's specific logout endpoint
  const logoutUrl = "https://{your-provider-domain}/logout";
  const postLogout = encodeURIComponent("https://myapp.com/goodbye");
  window.location.href = <span class="hljs-subst">${logoutUrl}</span>?post_logout_redirect_uri=<span class="hljs-subst">${postLogout}</span>;
}

At the end of the day, implicit flow is a relic. It served us well when apps were simpler, but with modern browser privacy and the rise of pkce, it's time to move on. Stay sharp, keep your dependencies updated, and always validate those tokens on the backend.