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Security Best Practices for Managing API Access Tokens


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Modern applications, bothweb-based and native, rely on APIs on the backend to access protected resources. To authorize access to those APIs, a request must include some kind of access token or key. This article focuses on security best practices for access token management ― for API providers and application developers alike.

Let's Talk About Trust First!

When dealing with security, a single rule prevails: trust no one. If you're an API provider, you can't trust that the application invoking the APIs is the one you expect, that the token you received has not been stolen, or that the communication between client and the server has not been intercepted . On the client-side, you can't trust that the application will not be decompiled (exposing embedded secrets), that the application storage will not be compromised through an XSS attack, or that your users are not being fooled into submitting forged requests .

This implies that you must put into place proper measures to securely obtain, store, and manage the security tokens required to invoke backend APIs.

Additionally, you may think your APIs are safe if you have never publicly advertised them. To you, they feel private because theyare only used by your enterprise applications. However, if they can be used from a mobile application, they are on the public internet, and thus public. Any API exposed outside your enterprise network must be considered public .

Obtaining Tokens an API Keys

When it comes to using an API, you are usually offered two choices: pass a static piece of information together with the API call or obtain that piece of information dynamically prior to invoking the API. This piece of information is usually an access token or API key. BasicAuth is still used for some APIs for legacy reasons but is deprecated as a mainstream solution.

When designing the security aspects of your API, you must choose wisely how your API consumers access it. As per usual with security measures, the induced risk is the key factor to take into account. Securing an API that only allows for consulting weather data is very different from securing a banking payments API.

While using an API key is easier for the developer, it does not give the same level of security as an access token obtained with two-factor user authentication and the proper identification of the client application. Moreover, an API key does not carry any information about the user and can't be used at the backend levelto decide which operations the API consumer is allowed to invoke. Finally, API keys never expire unless revoked by the API provider.

OAuth was created to address these drawbacks:

The application accessing the resource is known (using client application credentials).

The API provider can define scopes to limit the access to certain operations (you can GET a catalog entry, but you can't PUT a new catalog entry, even with a valid token).

Tokens have a limited lifetime.

Let's Start With Some Terminology

The OAuth terminology can sometimes be confusing. In the table below, we present a mapping from practical, development focused terminology to OAuth terminology.

Practical Name OAuth Terminology Description Application Client

The client is the application accessing a resource on behalf o fa user.

Web server apps Confidential client

An application running on the server side and capable of safely storing an application secret.

Single-page apps/browser-based apps/mobile apps Public client

An application entirely running on the client-side or on a device that cannot safely store an application secret.

API Resource server

The API is the means to access the resources belonging to the user (e.g. a bank account).

OAuth Server Authorization server

The OAuth server is in charge of processing the OAuth token management requests (authorize access, issue tokens, revoke tokens).

User Resource owner

The person granting access to the resource the application is trying to access.

Access token Bearer token

The access token authorizes the application to access the API.

Opaque vs. JWT

OAuth does not mandate the access token format, and as such, depending on the OAuth server implementation, the access token could be opaque (typically a long string carrying no information)or a JSON web token (JWT).

The key advantage with JWTs is the ability to contain claims, or information about the user, which the backend services can use to make business logic decisions.

Learning the OAuth Dance

OAuth grant types define how a client can obtain a token. Our team commonly refers to this as the "OAuth dance." There are many ways to dance in the OAuth world, but there is only one you must learn: authorization code. Other grant types can be useful in some circumstances, but the authorization code grant type is the recommended way to obtain an access token for all types of applications: web apps, native apps, and mobile apps.

For public clients and mobile apps, in particular, an additional security measure is recommended to prevent the theft of the authorization code. This security layer is described in the Proof Key for Code Exchange standard (PKCE, pronounced "pixy") . You can learn more about PKCE and how to use it here. If you are an API provider, make sure your OAuth server supports this option.

You should use special care with the Resource Owner password grant: being the simplest to implement, it's quite attractive. However, since its core requisite is client-server trust, you probably should

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