Security implications of adding all domains to CORS (Access-Control-Allow-Origin: *)

Question

It is said that instead of adding all domains to CORS, one should only add a set of domains. Yet it is sometimes not trivial to add a set of domains. E.g. if I want to publicly expose an API then for every domain that wants to make a call to that API I would need to be contacted to add that domain to the list of allowed domains.

I'd like to make a conscious trade off decision between security implications and less work.

The only security issues I see are DoS attacks and CSRF attacks. CSRF attacks can already be achieved with IMG elements and FORM elements. DoS attacks related to CORS can be overcome by blocking requests upon the referrer header.

Am I missing security implications?

===Edit===

• It is assumed that the Access-Control-Allow-Credentials Header is not set
• I know how to add a given list of domains "CORS access" and I'm therefore only interested in the security implications of adding all domains "CORS access"

Answer 1

Cross-Site Request Forgery attacks are far and away the primary concern that Access-Control-Allow-Origin addresses.

Ryan is certainly correct regarding content retrieval. However, on the subject of making the request there is more to say here. Many web sites now provide RESTful web services that expose a wide range of features that may involve making significant changes in the backend. Very often, these RESTful services are intended to be invoked with an XHR (e.g. AJAX) request (probably with a "Single Page Application" as the front-end). If a user has an active session granting access to these services when they visit a malicious third-party site, that site may try to invoke those REST endpoints behind the scenes, passing in values that could compromise the user or the site. Depending on how the REST services are defined, there are various ways to protect against this.

In the specific case of REST web services for a Single Page App, you can dictate that all requests to the backend REST endpoints are made with XHR and refuse any non-XHR request. You can dictate this by checking for the presence of a custom request header (something like jQuery's X-Requested-With). Only XHR-type requests can set these headers; simple GET and POST requests from forms and embedded resources cannot. Finally, the reason that we want to dictate XHR requests gets us back to the original question - XHR requests are subject to CORS rules.

If you allowed Access-Control-Allow-Origin: *, then any site could make any AJAX request on the user's behalf to your REST endpoints. If your REST endpoints involve any kind of sensitive data or allow for data persistence, then this is an unacceptable security vulnerability. Instead, enforce XHR-only requests like I described and define a whitelist of origins allowed to make those requests.

It's worth pointing out that if your REST endpoints do not expose any sensitive information, or if they don't allow the user to make any persistent data changes, then Access-Control-Allow-Origin: * may be the appropriate decision. Google Maps for instance provides read-only views into public map data; there is no reason to restrict the third party sites that may wish to invoke those services.