Best Practices for securing a REST API / web service

As tweakt said, Amazon S3 is a good model to work with. Their request signatures do have some features (such as incorporating a timestamp) that help guard against both accidental and malicious request replaying.

The nice thing about HTTP Basic is that virtually all HTTP libraries support it. You will, of course, need to require SSL in this case because sending plaintext passwords over the net is almost universally a bad thing. Basic is preferable to Digest when using SSL because even if the caller already knows that credentials are required, Digest requires an extra roundtrip to exchange the nonce value. With Basic, the callers simply sends the credentials the first time.

Once the identity of the client is established, authorization is really just an implementation problem. However, you could delegate the authorization to some other component with an existing authorization model. Again the nice thing about Basic here is your server ends up with a plaintext copy of the client's password that you can simply pass on to another component within your infrastructure as needed.

There are no standards for REST other than HTTP. There are established REST services out there. I suggest you take a peek at them and get a feel for how they work.

For example, we borrowed a lot of ideas from Amazon's S3 REST service when developing our own. But we opted not to use the more advanced security model based on request signatures. The simpler approach is HTTP Basic auth over SSL. You have to decide what works best in your situation.

Also, I highly recommend the book RESTful Web Services from O'reilly. It explains the core concepts and does provide some best practices. You can generally take the model they provide and map it to your own application.

You may also want to take a look at OAuth, an emerging open protocol for token-based authorization specifically targeting http apis.

It is very similar to the approach taken by flickr and remember the milk "rest" apis (not necessarily good examples of restful apis, but good examples of the token-based approach).

There is a great checklist found on Github:

Authentication

• Don't reinvent the wheel in Authentication, token generation, password storage. Use the standards.

• Use Max Retry and jail features in Login.

• Use encryption on all sensitive data.

JWT (JSON Web Token)

• Use a random complicated key (JWT Secret) to make brute forcing the token very hard.

• Don't extract the algorithm from the payload. Force the algorithm in the backend (HS256 or RS256).

• Make token expiration (TTL, RTTL) as short as possible.

• Don't store sensitive data in the JWT payload, it can be decoded easily.

OAuth

• Always validate redirect_uri server-side to allow only whitelisted URLs.

• Always try to exchange for code and not tokens (don't allow response_type=token).

• Use state parameter with a random hash to prevent CSRF on the OAuth authentication process.

• Define the default scope, and validate scope parameters for each application.

Access

• Limit requests (Throttling) to avoid DDoS / brute-force attacks.

• Use HTTPS on server side to avoid MITM (Man In The Middle Attack)

• Use HSTS header with SSL to avoid SSL Strip attack.

Input

• Use the proper HTTP method according to the operation: GET (read), POST (create), PUT/PATCH (replace/update), and DELETE (to delete a record), and respond with 405 Method Not Allowed if the requested method isn't appropriate for the requested resource.

• Validate content-type on request Accept header (Content Negotiation) to allow only your supported format (e.g. application/xml, application/json, etc) and respond with 406 Not Acceptable response if not matched.

• Validate content-type of posted data as you accept (e.g. application/x-www-form-urlencoded, multipart/form-data, application/json, etc).

• Validate User input to avoid common vulnerabilities (e.g. XSS, SQL-Injection, Remote Code Execution, etc).

• Don't use any sensitive data (credentials, Passwords, security tokens, or API keys) in the URL, but use standard Authorization header.

• Use an API Gateway service to enable caching, Rate Limit policies (e.g. Quota, Spike Arrest, Concurrent Rate Limit) and deploy APIs resources dynamically.

Processing

• Check if all the endpoints are protected behind authentication to avoid broken authentication process.

• User own resource ID should be avoided. Use /me/orders instead of /user/654321/orders.

• Don't auto-increment IDs. Use UUID instead.

• If you are parsing XML files, make sure entity parsing is not enabled to avoid XXE (XML external entity attack).

• If you are parsing XML files, make sure entity expansion is not enabled to avoid Billion Laughs/XML bomb via exponential entity expansion attack.

• Use a CDN for file uploads.

• If you are dealing with huge amount of data, use Workers and Queues to process as much as possible in background and return response fast to avoid HTTP Blocking.

• Do not forget to turn the DEBUG mode OFF.

Output

• Send X-Content-Type-Options: nosniff header.

• Send X-Frame-Options: deny header.

• Send Content-Security-Policy: default-src 'none' header.

• Remove fingerprinting headers - X-Powered-By, Server, X-AspNet-Version etc.

• Force content-type for your response, if you return application/json then your response content-type is application/json.

• Don't return sensitive data like credentials, Passwords, security tokens.

• Return the proper status code according to the operation completed. (e.g. 200 OK, 400 Bad Request, 401 Unauthorized, 405 Method Not Allowed, etc).

I've used OAuth a few times, and also used some other methods (BASIC/DIGEST). I wholeheartedly suggest OAuth. The following link is the best tutorial I've seen on using OAuth:

http://hueniverse.com/oauth/guide/

One of the best posts I've ever come across regarding Security as it relates to REST is over at 1 RainDrop. The MySpace API's use OAuth also for security and you have full access to their custom channels in the RestChess code, which I did a lot of exploration with. This was demo'd at Mix and you can find the posting here.

Thanks for the excellent advice. We ended up using a custom HTTP header to pass an identity token from the client to the service, in preparation for integrating our RESTful API with the the upcoming Zermatt Identity framework from Microsoft. I have described the problem here and our solution here. I also took tweakt's advice and bought RESTful Web Services - a very good book if you're building a RESTful API of any kind.

The fact that the SOAP world is pretty well covered with security standards doesn't mean that it's secure by default. In the first place, the standards are very complex. Complexity is not a very good friend of security and implementation vulnerabilities such as XML signature wrapping attacks are endemic here.

As for the .NET environment I won't help much, but “Building web services with Java” (a brick with ~10 authors) did help me a lot in understanding the WS-* security architecture and, especially, its quirks.