Is it safe to enable CORS to * for a public and readonly webservice?

Here’s something relevant from the Fetch spec (which defines CORS):

Basic safe CORS protocol setup

For resources where data is protected through IP authentication or a firewall (unfortunately relatively common still), using the CORS protocol is unsafe. (This is the reason why the CORS protocol had to be invented.)

However, otherwise using the following header is safe:

Access-Control-Allow-Origin: *

Even if a resource exposes additional information based on cookie or HTTP authentication, using the above header will not reveal it. It will share the resource with APIs such as XMLHttpRequest, much like it is already shared with curl and wget.

Thus in other words, if a resource cannot be accessed from a random device connected to the web using curl and wget the aforementioned header is not to be included. If it can be accessed however, it is perfectly fine to do so.

And the author of the Fetch/CORS spec goes into a bit more detail in a related blog posting:

It is completely safe to augment any resource with Access-Control-Allow-Origin: * as long as the resource is not part of an intranet (behind a firewall). In other words, a URL you can fetch from a server on the internet using wget or curl. For your basic web site this encompasses all resources on the site. The Access-Control-Allow-Origin header (part of CORS) tells the browser the resource can be shared.

Even if the resource includes confidential information based on cookies or HTTP authentication data in the request, including the header and sharing the resource is still safe, since the browser will make the request without any cookies or HTTP authentication data. And if the browser did make the request with cookies or HTTP authentication data, it would never share the resource because that would require an additional header, Access-Control-Allow-Credentials, and a different value for the aforementioned header.

So go ahead and safely share your public data with other applications!

If it is a public API then CORS should be enabled for all requests. One of the best security approach for public APIs is using app keys in request headers.