Common OAuth 2.0 Grant Types and When You Should Use Each One

Modern applications rely heavily on APIs, cloud services, mobile clients, and third-party integrations. In this environment, securely controlling access to protected resources is essential. OAuth 2.0 has become the standard framework for handling authorization across these systems.

OAuth 2.0 allows applications to access resources on behalf of a user or another system without exposing credentials. Instead of sharing passwords, applications request tokens that represent a specific set of permissions. These tokens are scoped, time-bound, and can be revoked, making OAuth 2.0 well suited for modern security requirements.

However, OAuth 2.0 is not a single flow. It defines multiple grant types, each designed for different application models and security assumptions. Selecting the correct grant type is critical. The wrong choice can increase complexity or introduce security risks, while the right choice simplifies implementation and improves long-term maintainability.

This article explains the most common OAuth 2.0 grant types, how they work, and when each one should be used.

Understanding OAuth 2.0 at a High Level

OAuth 2.0 is an authorization framework, not an authentication protocol. Its primary purpose is to allow a client application to access protected resources without directly handling user credentials.

Authentication, verifying who the user is, is typically handled using OpenID Connect (OIDC), which is built on top of OAuth 2.0. OAuth itself focuses on what access is granted, not who the user is.

OAuth 2.0 involves four main roles:

• Resource Owner: The user or system that owns the data
• Client: The application requesting access
• Authorization Server: Issues access tokens after successful authorization
• Resource Server: Hosts the protected APIs or data

Grant types define how the client obtains an access token from the authorization server.

Why OAuth 2.0 Uses Multiple Grant Types

Applications differ significantly in how they operate and what they can securely store:

• Some applications run entirely in the browser
• Some have secure backends
• Some operate without users
• Some run on devices with limited input capabilities

OAuth 2.0 grant types exist to address these differences. Each grant type assumes a specific level of trust and technical capability in the client. Using a grant type outside its intended context can lead to token exposure, weak security controls, or operational issues.

Understanding these differences helps teams design authorization flows that match their application architecture.

Authorization Code Grant

The Authorization Code Grant is the most commonly used OAuth 2.0 flow for applications that involve user interaction and have a backend server. It separates the user-facing authentication step from the token exchange process. This ensures that access tokens are never exposed to the browser or end user.

How the Flow Works

1. The user attempts to access the application
2. The client redirects the user to the authorization server
3. The user authenticates and grants consent
4. The authorization server redirects back with an authorization code
5. The client exchanges the code for an access token using a secure backend request
6. The access token is used to call protected APIs

Typical Use Cases

• Server-rendered web applications
• Applications with a backend-for-frontend pattern
• Systems that can securely store client secrets

Pros

• Strong security guarantees
• Tokens are not exposed in browser redirects
• Supports refresh tokens
• Widely supported and well documented

Cons

• Requires backend infrastructure
• Slightly more complex than simpler flows
  

When to Use It

Use the Authorization Code Grant when your application has a backend and requires a secure, long-lived session. For traditional web applications, this is usually the default choice.

Authorization Code Grant with PKCE

PKCE (Proof Key for Code Exchange) enhances the Authorization Code Grant for clients that cannot safely store secrets. It is now considered best practice for most public clients. PKCE protects against authorization code interception by binding the authorization request to the token exchange.

How PKCE Works

• The client generates a random code verifier
• A hashed version (the code challenge) is sent during authorization
• The original verifier must be provided during the token exchange
• If the verifier does not match, the token request fails
  
  

Typical Use Cases

• Single Page Applications (SPAs)
• Mobile applications
• Desktop applications
• Any public client

Pros

• Strong security without requiring a client secret
• Recommended by OAuth security best practices
• Works well with OpenID Connect

Cons

• Requires correct implementation
• Slightly more setup compared to older flows

When to Use It

Use Authorization Code with PKCE for all modern SPAs and mobile applications. It replaces older browser-based flows and provides significantly better security.

Implicit Grant

The Implicit Grant was originally designed for browser-based applications that could not perform backend token exchanges. In this flow, access tokens are returned directly in the redirect response.

Security Concerns

• Tokens are exposed in URLs
• Tokens can leak through browser history or logs
• No refresh tokens are supported
• Increased risk of token replay attacks
  

Current Status

Modern OAuth guidance strongly discourages the use of the Implicit Grant. Advances in browser security and the introduction of PKCE have made it unnecessary.

When to Use It

In new applications, the Implicit Grant should not be used. Authorization Code with PKCE is the recommended replacement.

Client Credentials Grant

The Client Credentials Grant is used for machine-to-machine communication where no user is involved. In this flow, the client authenticates as itself.

How the Flow Works

1. The client authenticates with the authorization server using its credentials
2. The authorization server issues an access token
3. The client uses the token to access protected APIs

Typical Use Cases

• Service-to-service communication
• Backend microservices
• Scheduled jobs or background processes
• Internal system integrations

Pros

• Simple and efficient
• No user interaction required
• Well suited for automation

Cons

• No user context
• Tokens represent the application, not an individual

When to Use It

Use the Client Credentials Grant when one system needs to access another system’s APIs without acting on behalf of a user.

Resource Owner Password Credentials Grant (ROPC)

The Resource Owner Password Credentials Grant allows the client to collect a user’s username and password and exchange them directly for a token.

Security Considerations

• The client handles raw credentials
• Incompatible with MFA and SSO
• Breaks the principle of credential isolation
• Increases risk if the client is compromised

Typical Use Cases

• Legacy applications
• Migration scenarios
• Highly trusted first-party systems

Advantages

• Simple flow
• No redirects required

Limitations

• High security risk
• Poor user experience
• Not suitable for modern identity systems

When to Use It

This grant should generally be avoided. If used, it should be limited to controlled environments and treated as a temporary solution during migration.

Device Authorization Grant (Device Code Flow)

The Device Authorization Grant is designed for devices that lack a browser or have limited input capabilities.

How the Flow Works

1. The device requests a device code from the authorization server
2. The user visits a verification URL on another device
3. The user authenticates and approves access
4. The device polls the authorization server
5. Once approved, the device receives an access token

Typical Use Cases

The Device Authorization Grant is widely used in situations where users cannot easily enter credentials on the device itself.

• Smart TVs & TV Apps
  
  • Streaming apps like Netflix and YouTube on Smart TVs commonly use this flow. The TV displays a short code, and users are prompted to visit a URL such as netflix.com/tv or youtube.com/activate on their phone or laptop to sign in and approve access.

• Streaming Devices
  
  • Devices such as Roku, Apple TV, and Amazon Fire TV rely on device code authentication to avoid typing usernames and passwords using a remote control.

• IoT & Smart Devices
  
  • Smart home hubs, connected displays, and other IoT devices use the device code flow to authenticate users through a separate, trusted device with a full browser.

• Command-Line Tools (CLI)
  
  • Developer tools and cloud CLIs use the Device Authorization Grant to authenticate users securely without embedding browsers or handling passwords in terminal-based environments.

These real-world examples mirror how popular consumer and developer products implement authentication for devices with limited input or display capabilities.

Advantages

• Secure authorization without local login
• Suitable for constrained environments

Limitations

• Slower than browser-based flows
• Requires polling logic
  
  

When to Use It

Use this grant when the device cannot perform standard redirects or display a login screen.

Refresh Tokens and Token Lifecycle

Refresh tokens allow applications to obtain new access tokens without re-authenticating the user. They are critical for maintaining sessions securely.

Key best practices include:

• Store refresh tokens securely
• Rotate refresh tokens when supported
• Revoke tokens on logout or compromise
• Avoid exposing refresh tokens to the browser unless explicitly supported

Choosing the Right Grant Type

The correct OAuth 2.0 grant type depends on:

• Application architecture
• Security requirements
• Presence of a backend
• User interaction
• Trust level of the client

General Guidance

• Web apps with backends: Authorization Code
• SPAs and mobile apps: Authorization Code with PKCE
• Service-to-service APIs: Client Credentials
• Devices with limited input: Device Authorization
• Legacy systems: Avoid ROPC where possible
• Implicit flow: Do not use

Common Implementation Mistakes

Even with the right grant type, issues can arise:

• Using client secrets in public clients
• Skipping PKCE for SPAs
• Storing tokens insecurely
• Requesting overly broad scopes
• Failing to rotate or revoke tokens

Careful implementation and adherence to best practices are essential.

Bottom Line

OAuth 2.0 provides a flexible and secure framework for managing access across modern applications, but its effectiveness depends on choosing the right grant type. Each grant is designed for a specific application model, trust level, and security requirement. Selecting the correct flow reduces risk, simplifies implementation, and supports long-term scalability.

For most user-facing applications, Authorization Code with PKCE is the recommended standard, while Client Credentials is best suited for service-to-service communication. Other grant types address specific scenarios, and older flows should be avoided in new designs.

If you want to simplify OAuth 2.0 implementation while following security best practices, Authgear offers a centralized identity platform with built-in support for modern OAuth and OpenID Connect flows. Authgear helps teams manage authorization across web apps, mobile apps, APIs, and microservices with consistent security and lower operational overhead.

Start your free trial of Authgear today to streamline OAuth 2.0 implementation and build secure, scalable access control across your applications.

FAQs

What is the most secure OAuth 2.0 grant type?

There is no single “most secure” grant for all scenarios, but Authorization Code with PKCE is considered the best option for most modern user-facing applications. It protects against token interception and works well for web, mobile, and SPA environments.

Should I still use the Implicit Grant for SPAs?

No. The Implicit Grant is no longer recommended. Modern best practices advise using Authorization Code with PKCE for SPAs, as it provides stronger security and better token management.

When should I use the Client Credentials Grant?

The Client Credentials Grant is best for machine-to-machine communication, such as microservices, backend jobs, or internal APIs. It should be used when no user context is required.

Is the Resource Owner Password Credentials Grant ever acceptable?

This grant should generally be avoided. It may appear in legacy systems or controlled migration scenarios, but it introduces security risks and does not support modern features like MFA or SSO.

Can I use multiple OAuth 2.0 grant types in one system?

Yes. Many organizations use multiple grant types depending on the application. For example, SPAs may use Authorization Code with PKCE, while backend services use Client Credentials. What matters is choosing the right grant for each use case and managing them consistently.