This post describes OAuth 2.0 in a simplified format to help developers and service providers implement the protocol.
The OAuth 2 spec can be a bit confusing to read, so I've written this post to help describe the terminology in a simplified format. The core spec leaves many decisions up to the implementer, often based on security tradeoffs of the implementation. Instead of describing all possible decisions that need to be made to successfully implement OAuth 2, this post makes decisions that are appropriate for most implementations.
Table of Contents
- Roles: Applications, APIs and Users
- Creating an App
- Authorization: Obtaining an access token
- Making Authenticated Requests
- Differences from OAuth 1.0
- Resources
Roles
The Third-Party Application: "Client"
The client is the application that is attempting to get access to the user's account. It needs to get permission from the user before it can do so.
The API: "Resource Server"
The resource server is the API server used to access the user's information.
The Authorization Server
This is the server that presents the interface where the user approves or denies the request. In smaller implementations, this may be the same server as the API server, but larger scale deployments will often build this as a separate component.
The User: "Resource Owner"
The resource owner is the person who is giving access to some portion of their account.
Creating an App
Before you can begin the OAuth process, you must first register a new app with the service. When registering a new app, you usually register basic information such as application name, website, a logo, etc. In addition, you must register a redirect URI to be used for redirecting users to for web server, browser-based, or mobile apps.
Redirect URIs
The service will only redirect users to a registered URI, which helps prevent some attacks. Any HTTP redirect URIs must be protected with TLS security, so the service will only redirect to URIs beginning with "https". This prevents tokens from being intercepted during the authorization process. Native apps may register a redirect URI with a custom URL scheme for the application, which may look like demoapp://redirect.
Client ID and Secret
After registering your app, you will receive a client ID and a client secret. The client ID is considered public information, and is used to build login URLs, or included in Javascript source code on a page. The client secret must be kept confidential. If a deployed app cannot keep the secret confidential, such as single-page Javascript apps or native apps, then the secret is not used, and ideally the service shouldn't issue a secret to these types of apps in the first place.
Authorization
The first step of OAuth 2 is to get authorization from the user. For browser-based or mobile apps, this is usually accomplished by displaying an interface provided by the service to the user.
OAuth 2 provides several "grant types" for different use cases. The grant types defined are:
- Authorization Code for apps running on a web server, browser-based and mobile apps
- Password for logging in with a username and password
- Client credentials for application access
- Implicit was previously recommended for clients without a secret, but has been superceded by using the Authorization Code grant with no secret.
Each use case is described in detail below.
Web Server Apps
Web server apps are the most common type of application you encounter when dealing with OAuth servers. Web apps are written in a server-side language and run on a server where the source code of the application is not available to the public. This means the application is able to use its client secret when communicating with the authorization server, which can help avoid some attack vectors.
Authorization
Create a "Log In" link sending the user to:
https://oauth2server.com/auth?response_type=code&
client_id=CLIENT_ID&redirect_uri=REDIRECT_URI&scope=photos&state=1234zyx- code - Indicates that your server expects to receive an authorization code
- client_id - The client ID you received when you first created the application
- redirect_uri - Indicates the URI to return the user to after authorization is complete
- scope - One or more scope values indicating which parts of the user's account you wish to access
- state - A random string generated by your application, which you'll verify later
The user sees the authorization prompt
If the user clicks "Allow," the service redirects the user back to your site with an auth code
https://oauth2client.com/cb?code=AUTH_CODE_HERE&state=1234zyx- code - The server returns the authorization code in the query string
- state - The server returns the same state value that you passed
You should first compare this state value to ensure it matches the one you started with. You can typically store the state value in a cookie or session, and compare it when the user comes back. This ensures your redirection endpoint isn't able to be tricked into attempting to exchange arbitrary authorization codes.
Token Exchange
Your server exchanges the auth code for an access token:
POST https://api.oauth2server.com/token
grant_type=authorization_code&
code=AUTH_CODE_HERE&
redirect_uri=REDIRECT_URI&
client_id=CLIENT_ID&
client_secret=CLIENT_SECRET- grant_type=authorization_code - The grant type for this flow is authorization_code
- code=AUTH_CODE_HERE - This is the code you received in the query string
- redirect_uri=REDIRECT_URI - Must be identical to the redirect URI provided in the original link
- client_id=CLIENT_ID - The client ID you received when you first created the application
- client_secret=CLIENT_SECRET - Since this request is made from server-side code, the secret is included
The server replies with an access token and expiration time
{
"access_token":"RsT5OjbzRn430zqMLgV3Ia",
"expires_in":3600
}or if there was an error
{
"error":"invalid_request"
}Security: Note that the service must require apps to pre-register their redirect URIs.
Single-Page Apps
Single-page apps (or browser-based apps) run entirely in the browser after loading the source code from a web page. Since the entire source code is available to the browser, they cannot maintain the confidentiality of their client secret, so the secret is not used in this case. The flow is exactly the same as the authorization code flow above, but at the last step, the authorization code is exchanged for an access token without using the client secret.
Authorization
Create a "Log In" link sending the user to:
https://oauth2server.com/auth?response_type=code&
client_id=CLIENT_ID&redirect_uri=REDIRECT_URI&scope=photos&state=1234zyx- code - Indicates that your server expects to receive an authorization code
- client_id - The client ID you received when you first created the application
- redirect_uri - Indicates the URI to return the user to after authorization is complete
- scope - One or more scope values indicating which parts of the user's account you wish to access
- state - A random string generated by your application, which you'll verify later
The user sees the authorization prompt
If the user clicks "Allow," the service redirects the user back to your site with an auth code
https://oauth2client.com/cb?code=AUTH_CODE_HERE&state=1234zyx- code - The server returns the authorization code in the query string
- state - The server returns the same state value that you passed
You should first compare this state value to ensure it matches the one you started with. You can typically store the state value in a cookie, and compare it when the user comes back. This ensures your redirection endpoint isn't able to be tricked into attempting to exchange arbitrary authorization codes.
Token Exchange
POST https://api.oauth2server.com/token
grant_type=authorization_code&
code=AUTH_CODE_HERE&
redirect_uri=REDIRECT_URI&
client_id=CLIENT_ID- grant_type=authorization_code - The grant type for this flow is authorization_code
- code=AUTH_CODE_HERE - This is the code you received in the query string
- redirect_uri=REDIRECT_URI - Must be identical to the redirect URI provided in the original link
- client_id=CLIENT_ID - The client ID you received when you first created the application
Mobile Apps
Like browser-based apps, mobile apps also cannot maintain the confidentiality of their client secret. Because of this, mobile apps must also use an OAuth flow that does not require a client secret. There are some additional concerns that mobile apps should keep in mind to ensure the security of the OAuth flow.
Authorization
Create a "Log in" button sending the user to either the native app of the service on the phone, or a mobile web page for the service. On iPhone, apps can register a custom URI protocol such as "facebook://" so the native Facebook app is launched whenever a URL with that protocol is visited. On Android, apps can register URL matching patterns which will launch the native app if a URL matching the pattern is visited.
Using the Service's Native App
If the user has the native Facebook app installed, direct them to the following URL:
fbauth2://authorize?response_type=code&client_id=CLIENT_ID
&redirect_uri=REDIRECT_URI&scope=email&state=1234zyx- response_type=code - indicates that your server expects to receive an authorization code
- client_id=CLIENT_ID - The client ID you received when you first created the application
- redirect_uri=REDIRECT_URI - Indicates the URI to return the user to after authorization is complete, such as
fb00000000://authorize - scope=email - One or more scope values indicating which parts of the user's account you wish to access
- state=1234zyx - A random string generated by your application, which you'll verify later
For servers that support the PKCE extension (and if you're building a server, you should support the PKCE extension), you'll also include the following parameters. First, create a "code verifier" which is a random string that the app stores locally.
- code_challenge=XXXXXXX - This is a base64-encoded version of the sha256 hash of the code verifier string
- code_challenge_method=S256 - Indicates the hashing method used to compute the challenge, in this case, sha256.
Note that your redirect URI will probably look like fb00000000://authorize where the protocol is a custom URL scheme that your app has registered with the OS.
Using a Web Browser
If the service does not have a native application, you can launch a mobile browser to the standard web authorization URL. Note that you should never use an embedded web view in your own application, as this provides the user no guarantee that they are actually are entering their password in the service's website rather than a phishing site.
You should either launch the native mobile browser, or use the new iOS "SafariViewController" to launch an embedded browser in your application. This API was added in iOS 9, and provides a mechanism to launch a browser inside the application that both shows the address bar so the user can confirm they're on the correct website, and also shares cookies with the real Safari browser. It also prevents the application from inspecting and modifying the contents of the browser, so can be considered secure.
https://facebook.com/dialog/oauth?response_type=code&client_id=CLIENT_ID
&redirect_uri=REDIRECT_URI&scope=email&state=1234zyxAgain, if the service supports PKCE, then those parameters should be included as well as described above.
- response_type=code - indicates that your server expects to receive an authorization code
- client_id=CLIENT_ID - The client ID you received when you first created the application
- redirect_uri=REDIRECT_URI - Indicates the URI to return the user to after authorization is complete, such as
fb00000000://authorize - scope=email - One or more scope values indicating which parts of the user's account you wish to access
- state=1234zyx - A random string generated by your application, which you'll verify later
The user will see the authorization prompt
Token Exchange
After clicking "Approve", the user will be redirected back to your application with a URL like
fb00000000://authorize#code=AUTHORIZATION_CODE&state=1234zyxYour mobile application should first verify that the state corresponds to the state that was used in the initial request, and can then exchange the authorization code for an access token.
The token exchange will look the same as exchanging the code in the web server app case, except that the secret is not sent. If the server supports PKCE, then you will need to include an additional parameter as described below.
POST https://api.oauth2server.com/token
grant_type=authorization_code&
code=AUTH_CODE_HERE&
redirect_uri=REDIRECT_URI&
client_id=CLIENT_ID&
code_verifier=VERIFIER_STRING- grant_type=authorization_code - The grant type for this flow is authorization_code
- code=AUTH_CODE_HERE - This is the code you received in the query string
- redirect_uri=REDIRECT_URI - Must be identical to the redirect URI provided in the original link
- client_id=CLIENT_ID - The client ID you received when you first created the application
- code_verifier=VERIFIER_STRING - The plaintext string that you previously hashed to create the code_challenge
The authorization server will verify this request and return an access token.
If the server supports PKCE, then the authorization server will recognize that this code was generated with a code challenge, and will hash the provided plaintext and confirm that the hashed version corresponds with the hashed string that was sent in the initial authorization request. This ensures the security of using the authorization code flow with clients that don't support a secret.
Other Grant Types
Password
OAuth 2 also provides a "password" grant type which can be used to exchange a username and password for an access token directly. Since this obviously requires the application to collect the user's password, it must only be used by apps created by the service itself. For example, the native Twitter app could use this grant type to log in on mobile or desktop apps.
To use the password grant type, simply make a POST request like the following:
POST https://api.oauth2server.com/token
grant_type=password&
username=USERNAME&
password=PASSWORD&
client_id=CLIENT_ID- grant_type=password - The grant type for this flow is password
- username=USERNAME - The user's username as collected by the application
- password=PASSWORD - The user's password as collected by the application
- client_id=CLIENT_ID - The client ID you received when you first created the application
The server replies with an access token in the same format as the other grant types.
Note, the client secret is not included here under the assumption that most of the use cases for password grants will be mobile or desktop apps, where the secret cannot be protected.
Application access
In some cases, applications may need an access token to act on behalf of themselves rather than a user. For example, the service may provide a way for the application to update their own information such as their website URL or icon, or they may wish to get statistics about the users of the app. In this case, applications need a way to get an access token for their own account, outside the context of any specific user. OAuth provides the client_credentials grant type for this purpose.
To use the client credentials grant type, make a POST request like the following:
POST https://api.oauth2server.com/token
grant_type=client_credentials&
client_id=CLIENT_ID&
client_secret=CLIENT_SECRETThe response will include an access token in the same format as the other grant types.
Making Authenticated Requests
The end result of all the grant types is obtaining an access token.
Now that you have an access token, you can make requests to the API. You can quickly make an API request using cURL as follows:
curl -H "Authorization: Bearer RsT5OjbzRn430zqMLgV3Ia" \
https://api.oauth2server.com/1/me
That's it! Make sure you always send requests over HTTPS and never ignore invalid certificates. HTTPS is the only thing protecting requests from being intercepted or modified.
Differences from OAuth 1.0
OAuth 1.0 was largely based on existing proprietary protocols such as Flickr's "FlickrAuth" and Google's "AuthSub". The result represented the best solution based on actual implementation experience. However, after several years of working with the protocol, the community learned enough to rethink and improve the protocol in three main areas where OAuth 1.0 proved limited or confusing:
Authentication and Signatures
The majority of developers' confusion and annoyance with OAuth 1.0 was due to the cryptographic requirements of signing requests with the client ID and secret. Losing the ability to easily copy and paste cURL examples made it much more difficult to get started quickly.
OAuth 2 recognizes this difficulty and replaces signatures with requiring HTTPS for all communications between browsers, clients and the API.
User Experience and Alternative Authorization Flows
OAuth includes two main parts, obtaining an access token, and using the access token to make requests. OAuth 1.0 works best for desktop web browsers, but fails to provide a good user experience for native desktop and mobile apps or alternative devices such as game or TV consoles.
OAuth 2 supports a better user experience for native applications, and supports extending the protocol to provide compatibility with future device requirements.
Performance at Scale
As larger providers started to use OAuth 1.0, the community quickly realized the protocol does not scale well. Many steps require state management and temporary credentials, which require shared storage and are difficult to synchronize across data centers. OAuth 1.0 also requires that the API server has access to the application's ID and secret, which often breaks the architecture of most large providers where the authorization server and API servers are completely separate.
OAuth 2 supports the separation of the roles of obtaining user authorization and handling API calls. Larger providers needing this scalability are free to implement it as such, and smaller providers can use the same server for both roles if they wish.
Resources
- Learn more about creating OAuth 2.0 Servers
- PKCE Extension
- Recommendations for Native Apps
- More information is available on OAuth.net
- Some content adapted from hueniverse.com.
Previous versions of this post:
OAuth Consultation
Aaron Parecki is available to provide OAuth training and consultation for you and your team. A typical consultation will involve a half-day or full-day session, starting with an introduction to OAuth 2.0 tailored to your needs, followed by spending time addressing the specific questions of your team relating to your product. Please email me at aaron@parecki.com for my availability.