Introduction
Access control refers to the cybersecurity practice of determining whether a user, API, application, or device (requesting party) should have access to a resource, system, or service or perform an action. This process is called authentication (verifying the identity of the requesting party) and authorization (determining their access rights).
OAuth 2.0 has become the go-to protocol for authorization and access delegation, which facilitates fast and secure authentication and authorization for users to APIs, servers, devices, and apps. This is done without sharing any user credentials, like passwords. Instead, access tokens are used to prove an identity, keeping user credentials safe. By using a centralized OAuth server to issue tokens, you can achieve fine-grained access control at the API level for robust API security. This article will go through the basics of OAuth and how to implement it to secure API access.
OAuth Flow Overview
There are four different parties in the OAuth 2.0 specification published in the RFC 6749 document:
Resource owner: refers to the entity that grants access to the protected resource, which is often an end-user.
Client: refers to the system or application that requests access on behalf of the resource owner (typically an app requesting access).
Authorization server: the entity that authenticates the resource owner and issues OAuth tokens to the client.
Resource server: refers to the entity hosting the protected resource, which is often a backend API. It protects its data via access tokens, which are issued by the authorization server on behalf of the client.
Once you understand the key roles in OAuth, the next step is choosing the right flow based on how your application interacts with users and systems.
Choosing the Right OAuth Flow
To choose the right OAuth flow, you need to consider the characteristics of the application and the use case. Authorization flows in OAuth 2.0 fall into two categories: user-centric flows that involve end-user authentication and machine-to-machine flows where no user is involved. In this case, the client credential flow should be used to get an access token that represents the client, as no user authentication is involved in the process.
When users have to be authenticated by the authorization server, the client must choose the correct user interface. There are two approaches to trigger user interactions:
Same-device authentication: If the device that runs the client application can also open a browser, and the user can conveniently operate the device (like enter their password), then you should use the code flow. It allows both mobile and web clients to obtain tokens securely and gain access to APIs. The code flow uses a redirect approach, where the user is redirected in the browser to the authorization server for authentication. For security reasons, the authorization server never returns tokens directly to the client but sends a temporary grant — an authorization code. The client must subsequently exchange the code for a set of tokens: an access token and, optionally, an ID token and a refresh token.
Cross-Device Authentication: In some cases, such as with smart TVs, IoT devices, or remote consoles, users can't authenticate directly on the device they're interacting with due to limited input or browser support. If the user can still interact with the client and the client can present a code and link (or a QR code) for the user to authenticate on a separate device, then the device flow is a suitable option for this decoupled authentication scenario.
If the user needs to be authenticated remotely and the client cannot communicate the authentication information to the user, you should use the client-initiated backchannel authentication (CIBA) flow. It allows the client to remotely initiate an authorization flow that the user can complete on their own device. As the client needs to initiate the flow for a concrete user, CIBA requires that the user is already registered with the authorization server through an out-of-band process, and the client must know the user’s identifier beforehand.
In general, the most recommended flow for the majority of cases is the OAuth code flow in combination with Proof Key for Code Exchange (PKCE). It enhances the security of the authorization code flow, ensuring that the application initiating the authorization flow is the same one that finishes it.
Handling Tokens
Once an OAuth flow completes, the client gets a set of tokens to use. One of them is the access token that allows the client to call APIs and access the user’s data. As the access token is a powerful credential, it should always be kept safe. While it’s simpler for backend services and mobile apps to keep such a token secure, browser-based applications have a much more difficult task. JavaScript applications are susceptible to cross-site scripting (XSS) attacks, which can steal the token. The best practice is to keep the token out of the browser altogether and utilize secure cookies instead, through solutions like the token handler pattern.
Implementing OAuth Step-by-Step
Once you’ve picked the right flow, here’s how to actually implement OAuth. The steps below cover the main building blocks for implementing OAuth, but they are not exhaustive. Depending on your security requirements and particular use case, you’ll likely need to explore the topic deeper, but hopefully this will be enough to get you started.
1. Set Up an Authorization Server
The first step is to set up an authorization server, such as the Curity Identity Server, Okta, or Auth0. This acts as the central component responsible for issuing and managing tokens, handling user authentication, and enforcing security policies. The authorization server must be configured according to your specific requirements and use cases.
*2. Register Your Client Application
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Now it’s time to register your app with the authorization server. This usually involves generating a client ID and secret, setting up redirect URIs, and choosing which grant types and scopes your app will use. It's also here that scopes play a crucial role — they define the level of access your client is requesting, and when combined with a consent screen, they ensure users are informed and in control of what permissions they’re granting.
3. Obtain and Use Access Tokens
With your client registered, your app can begin requesting access tokens. The flow you use depends on the nature of your application. Once access tokens are obtained, they allow the client to make authenticated requests to resource servers. To maintain security, follow token expiration and refresh best practices: use short-lived access tokens and secure, time-limited refresh tokens to minimize the impact of a compromised token.
4. Validate and Introspect Tokens
Before trusting any access token, you must validate it. This can involve checking the token’s signature and claims (in the case of JWTs) or performing token introspection against the authorization server. Validation ensures the token is still active, correctly scoped, and has not been tampered with or revoked.
*5. Secure the Token *
Once you have an end-to-end flow up and running, you can further enhance the security of your solution. All token-related communication should happen over HTTPS to prevent interception. Store tokens and credentials securely and apply logging and monitoring to detect unusual activity. You can further strengthen security by implementing token binding and client attestation. These techniques ensure that only the authorized client can use access tokens, preventing token theft and replay attacks by binding the token to the client's identity or device.
Conclusion
OAuth 2.0 provides a flexible and secure framework for managing API access control in modern applications. Selecting the right flow for your specific use case is crucial for maintaining usability and security. Ultimately, OAuth isn’t just about obtaining access tokens — it's about building trust between users, clients, and APIs. A well-designed OAuth implementation puts security first while also delivering a smooth and secure user experience that scales with your system's needs.
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