Introduction

Ever signed into a new cool app using your Google or Facebook account with just a click? It feels seamless, almost like magic. You didn’t have to create a new password, yet the new app somehow got permission to access some of your information (like your profile picture or email) without ever seeing your Google or Facebook password. This digital handshake is powered by a framework called OAuth 2.0, and the different ways this handshake can happen are known as “grant types.”

But why so many “ways”? Isn’t one good enough? Imagine you have a house (your data). You might give a spare key to a family member (full trust), a temporary code to a cleaner (limited access, specific time), or just open the door yourself for a delivery person (very limited, supervised access). OAuth 2.0 grant types are like these different methods of granting access, each suited for different scenarios and levels of trust.

In this blog, we’ll dive deep into the world of OAuth 2.0 grant types — the mechanisms that power secure authorization for modern apps. We’ll focus on two of the most talked-about flows: the Authorization Code flow and the (now deprecated) Implicit flow. If you’ve ever wondered how secure integrations with APIs like Google, Slack, or GitHub actually work under the hood, this guide will help connect those dots in a developer-friendly way.

OAuth 2.0 Explained: How Apps Safely Share Permissions

What is OAuth 2.0?

OAuth 2.0 is an open standard for access delegation, commonly used as a way for Internet users to grant websites or applications access to their information on other websites but without giving them the passwords. Think of it as a valet key for your car; it allows someone to drive your car but doesn’t give them access to the trunk or glove compartment, and they certainly don’t get your house keys!

It’s crucial to note that OAuth 2.0 is about authorization (what an application is allowed to do on your behalf) and not authentication (proving who you are). Though often used together, they are distinct concepts. You first authenticate with the service that holds your data (e.g., Google), then you authorize the third-party app to access specific parts of that data.

Key players in an OAuth 2.0 flow:

•  Resource Owner: You! The user who owns the data and can grant access.
•  Client: The third-party application (e.g., the new cool app) that wants to access your data.
•  Authorization Server: The server that authenticates you and issues access tokens after you grant permission (e.g., Google’s or Facebook’s authentication server).
•  Resource Server: The server hosting your protected data (e.g., Google Drive API, Facebook Graph API).

Why So Many Grant Types?

What are OAuth grant types?

OAuth grant types are the different methods or “flows” an application (the Client) can use to get an access token from an Authorization Server. This access token is the key that allows the Client to access specific resources on the Resource Server on behalf of the Resource Owner (you).

The reason for multiple grant types is simple: different types of applications have different capabilities and security requirements. A powerful web application with a secure backend server can handle more complex and secure flows than a simple JavaScript application running entirely in a user’s browser.

The OAuth 2.0 framework (RFC 6749) defines several grant types, but we’ll focus on the most common ones, particularly Authorization Code and Implicit.

The Main Characters: Common OAuth 2.0 Grant Types

Let’s meet the cast:

1. Authorization Code Grant: The workhorse, ideal for traditional web apps with a backend and now, with PKCE, for mobile and single-page apps too.
2. Implicit Grant: A simplified flow once used for browser-based or mobile apps, but now largely discouraged due to security concerns.
3. Resource Owner Password Credentials (ROPC) Grant: The user provides their username and password directly to the client application. Use with extreme caution, only for highly trusted or legacy applications.
4. Client Credentials Grant: Used for machine-to-machine authentication, where the client is accessing its own resources or resources it controls.
5. Refresh Token Grant: Not a primary grant type for initial authorization, but a mechanism to obtain a new access token when the current one expires.

Now, let’s dive deeper into the two grant types often compared: Authorization Code and Implicit.

How the Authorization Code Grant Works

Imagine you want to allow a trusted courier service (the Client application) to pick up a specific package (your data) from a high-security vault (the Resource Server). You (the Resource Owner) don’t want to give the courier the master key to the vault (your password).

Instead, you go to the vault’s security office (the Authorization Server). You prove who you are, and tell them to give the courier a special, one-time-use slip (the authorization code). The courier takes this slip back to their own secure office (the Client’s backend server) and exchanges it with the vault’s security office for a temporary access badge (the access token). Now, the courier can access only the package you authorized.

How it works (simplified):

1. User Initiates: You click “Login with Google” on CoolApp.com.
2. Redirect to Authorize: CoolApp.com redirects your browser to Google’s Authorization Server.
3. User Authenticates & Authorizes: You log into Google (if not already) and approve CoolApp.com’s request for specific permissions (e.g., “view your email address”).
4. Authorization Code Issued: Google redirects your browser back to CoolApp.com with an authorization_code in the URL.
5. Token Exchange (Secure Backend Channel): CoolApp.com’s backend server takes this authorization_code and, using its own confidential client_secret, exchanges it directly with Google’s Authorization Server for an access_token (and often a refresh_token). This step is hidden from the browser.
6. Access Protected Resource: CoolApp.com now uses the access_token to request your email address from Google’s Resource Server (e.g., Google People API).

Authorization Code Flow:

Pros of Authorization Code Grant:

•  Most Secure for Confidential Clients: The access token is transmitted directly to the client’s backend and never exposed to the user’s browser, reducing the risk of leakage.
•  Supports Refresh Tokens: Allows clients to obtain new access tokens without repeatedly asking the user for permission.
•  Client Authentication: The client authenticates itself using a client secret during the token exchange, adding an extra layer of security.

Cons of Authorization Code Grant:

•  More Complex: Involves more steps and requires a backend component to securely store the client secret and handle the token exchange.

When to Use It:

•  Traditional server-side web applications.
•  Mobile and Single-Page Applications (SPAs) when combined with PKCE (Proof Key for Code Exchange). PKCE is an extension described in RFC 7636 that mitigates authorization code interception attacks, making this flow secure even for public clients that cannot keep a secret.

How the Implicit Grant Works

Imagine you’re at a public kiosk (a JavaScript app in your browser) that needs to quickly show some of your non-sensitive information. You authorize it, and it immediately gets a short-lived pass (the access token) directly. No complex exchanges, just a quick grant.

This was the original idea behind the Implicit grant, designed for simplicity in client-side applications (like Single-Page Applications or SPAs) that don’t have a backend server to securely store a client secret.

How it worked (simplified):

1. User Initiates: You click “Login with ServiceX” on a SPA.
2. Redirect to Authorize: The SPA redirects your browser to ServiceX’s Authorization Server.
3. User Authenticates & Authorizes: You log in and approve the SPA’s request.
4. Access Token Issued Directly: ServiceX’s Authorization Server redirects your browser back to the SPA, but this time, the access_token is included directly in the URL fragment (the part after the #). The SPA’s JavaScript can then extract this token.
5. Access Protected Resource: The SPA uses the access_token to request your data.

Notice the absence of an authorization code and the backend exchange. The token goes straight to the browser.

Pros of Implicit Grant:

•  Simpler Flow: Fewer steps compared to the Authorization Code grant.
•  Suitable for Static Apps: Could be used by applications without a backend server.

Cons of Implicit Grant:

•  Less Secure: The access token is exposed in the browser’s URL and can be leaked through browser history, referrer headers, or malicious browser extensions.
•  No Refresh Tokens: Historically, the Implicit grant did not support refresh tokens, meaning the user would have to re-authenticate more frequently.
•  Vulnerable to Token Theft: Susceptible to attacks like Cross-Site Scripting (XSS) that could steal the token.

When to Use It (Historically):

•  Previously recommended for JavaScript-heavy SPAs or mobile applications that couldn’t protect a client secret.
•  Important Security Note: The Implicit grant is no longer recommended for new applications. The OAuth 2.0 Security Best Current Practice document strongly advises against using the Implicit grant due to its inherent security risks. Instead, Authorization Code grant with PKCE is the recommended approach for SPAs and mobile apps.

Authorization Code vs. Implicit Flow: The Showdown

Let’s put them side-by-side:

Why the Shift for SPAs? The Rise of Authorization Code with PKCE

PKCE allows “public” clients (like SPAs or mobile apps that cannot securely store a client secret) to use the Authorization Code flow securely. It adds a dynamic secret generated by the client for each authorization request. This secret is used to verify that the client requesting the access token is the same one that initiated the authorization request, preventing authorization code interception attacks.

This means SPAs can now benefit from the superior security of the Authorization Code flow without needing a backend to hide a static client secret. This is why the Authorization Code grant with PKCE is now the gold standard for SPAs and mobile applications.

A Quick Glance at Other Grant Types

While Authorization Code with PKCE covers many user-facing scenarios, other grant types serve specific purposes:

• Client Credentials Grant:
  •  Use Case: Machine-to-machine (M2M) communication. Think of two backend services talking to each other. The client application is acting on its own behalf, not on behalf of a user.
  •  Example: A reporting service fetching data from an analytics API. The reporting service uses its own client_id and client_secret to get an access token.
• Resource Owner Password Credentials (ROPC) Grant:
  •  Use Case: The user provides their username and password directly to the client application, which then sends them to the Authorization Server to get an access token.
  •  Why it’s risky: It trains users to enter their credentials into third-party apps, breaking the core principle of OAuth (not sharing passwords). It also doesn’t support multi-factor authentication easily.
  •  When to (cautiously) use: Only for highly trusted, first-party applications or during migration from legacy systems. Avoid for new developments. OWASP provides good guidance on authentication security.
• Refresh Token Grant:
  •  Use Case: Not for initial authorization, but used by clients to obtain a new access token when the current one expires. This avoids repeatedly prompting the user for credentials. It’s typically used in conjunction with the Authorization Code grant.

Choosing the Right OAuth 2.0 Grant Type

Selecting the appropriate grant type is critical for the security and user experience of your application. Here’s a simplified guide:

•  Server-Side Web Applications (with a backend): Use the Authorization Code grant. Your backend can securely store the client secret.
•  Single-Page Applications (SPAs) / Client-Side Web Apps: Use the Authorization Code grant with PKCE.
•  Native Mobile Applications: Use the Authorization Code grant with PKCE.
•  Machine-to-Machine (M2M) Applications / APIs: Use the Client Credentials grant.
•  Legacy or Highly Trusted First-Party Apps (with caution): ROPC might be considered, but strive to migrate to safer flows.
•  For New Developments: Avoid the Implicit grant and generally avoid ROPC.

Platforms like SSOJet provide SDKs and services that simplify implementing OAuth 2.0 correctly, often guiding you towards the most secure grant type for your application type.

The Future of OAuth: Towards Simpler Security

The world of digital security is always evolving. The OAuth working group is continuously refining best practices. An upcoming specification, OAuth 2.1, aims to consolidate these best practices by formally deprecating insecure flows like Implicit and ROPC (unless specific conditions are met), and making PKCE mandatory for the Authorization Code grant used by public clients. This simplification will make it easier for developers to build secure applications. You can follow developments on the official OAuth website.

Conclusion: Granting Access, Wisely

OAuth 2.0 grant types might seem complex at first, but they offer a flexible and standardized way to handle authorization in a wide range of applications. Understanding the “why” behind each grant type, especially the shift from Implicit to Authorization Code with PKCE for client-side apps, is key to building secure and user-friendly experiences.

By choosing the right grant type, you’re not just implementing a feature; you’re building trust with your users, assuring them that their data is handled responsibly. So, the next time you “Sign in with Google,” you’ll know a bit more about the sophisticated yet elegant dance of permissions happening behind the scenes.

Further Reading & Resources

•  The OAuth 2.0 Authorization Framework (RFC 6749): https://datatracker.ietf.org/doc/html/rfc6749
•  Proof Key for Code Exchange (PKCE) (RFC 7636): https://datatracker.ietf.org/doc/html/rfc7636
•  OAuth 2.0 Security Best Current Practice: https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics
•  OAuth.net: The official OAuth community site with news and resources: https://oauth.net/
•  DigitalOcean – An Introduction to OAuth 2: A good overview: https://www.digitalocean.com/community/tutorials/an-introduction-to-oauth-2

*** This is a Security Bloggers Network syndicated blog from SSOJet authored by Diksha Pooniya. Read the original post at: https://ssojet.com/blog/what-are-oauth-grant-types/