Authenticating a client-side web service request in a cached environment

Question

We're building a set of external web services to be consumed client-side (using jquery/AJAX) by visitors to our site. The web services need to be publicly available but we'd like to limit access to site visitors.

Importantly, the site in question sits behind a CDN and we cache page content for 24 hours; AJAX requests would preferably be cached as well but I'm conscious doing so will limit our authentication options. Our visitors access the site and services anonymously.

What are some standard "patterns" for authenticating client requests? I'm not dealing with confidential data per-se but do want to deter other users/sites from hijacking these services for liability (think data distribution) and performance reasons.

I'm thinking of a shared secret that's refreshed daily and used site-wide by all clients; any web service request would include the secret. Pretty basic but are there other, better ways for the service to detect the caller's origin in a manner that can't be spoofed?

Answer 1

If the threat to your web service is related to someone automating the client calls, you can implement rate limiting on server side. As you rightly mentioned, client can be required to provide key for each request. Alternatively, if only mortals are going to interact with web service, you can also implement Human Interaction Proof like Captcha etc. One thing to make sure is that "key" which will be used by client needs to given in controlled manner. I once came across a system which basically gave away unlimited keys - this means that automation control will be ineffective as an attacker can request as many keys and make unlimited calls. If you are limiting using IP address, make sure that you throttle requests on network part of ip address (A.B.C.X) as host part (X) can change (when users are behind proxies) If your clients are anonymous, the best/closest "identifier" is indeed address.