I have a .Net Core REST API. and a native mobile app (also a SPA).
I don't have any individual user authentication for my application. It's like a free service.
But I want to make sure that the service can be accessed only if the end-users are trying to access it from the native mobile app or the SPA. if the paste the GET/POST link on a browser, it should not be accessible.
Is it possible?
(If not a completely secure solution? at least a simple solution (where the system cannot be easily cracked)? BTW currently using a simple API-Key)
I don't have any individual user authentication for my application. It's like a free service.
User authentication only identifies who is in the request being made to the API server, not what is doing it.
This is a misconception that usually I find among developers of any seniority, thus I will recommend you to read this section from the article I wrote with the title Why Does Your Mobile App Need An Api Key?. The section explains in detail the difference between who and what is accessing your API server, but I will extract here the main takes from it:
The what is the thing making the request to the API server. Is it really a genuine instance of your mobile app, or is it a bot, an automated script or an attacker manually poking around your API server with a tool like Postman?
The who is the user of the mobile app that we can authenticate, authorize and identify in several ways, like using OpenID Connect or OAUTH2 flows.
In resume you need to think about the who as the user your API server will be able to Authenticate and Authorize access to the data, and to think about the what as the software making that request in behalf of the user.
But I want to make sure that the service can be accessed only if the end-users are trying to access it from the native mobile app or the SPA. if the paste the GET/POST link on a browser, it should not be accessible.
Is it possible?
"Yes" and "No"... Well I don't like to categorical use any of these affirmations, because in software is always more like it depends!, thus I surround them in quotes ;)
For a mobile app you can use the Mobile App Attestation concept to give an high degree of confidence that what is making the request is indeed the original and untampered version of your mobile app, and to learn more about it I recommend you to read this answer I gave to the question How to secure an API REST for mobile app?, specially the sections Securing the API Server and A Possible Better Solution.
For a web app all an attacker needs to do is to hit F12 to inspect the web page and view all the source code on it, therefore no direct solution here to tackle the issue of knowing with an high degree of confidence what is doing the request, but you can make the attacker life harder. I recommend you to read my answer to the question Secure api data from calls out of the app, specially the section dedicated to Defending the API Server.
In any response to a security question I always like to reference the excellent work from the OWASP foundation.
The OWASP API Security Project seeks to provide value to software developers and security assessors by underscoring the potential risks in insecure APIs, and illustrating how these risks may be mitigated. In order to facilitate this goal, the OWASP API Security Project will create and maintain a Top 10 API Security Risks document, as well as a documentation portal for best practices when creating or assessing APIs.
OWASP Mobile Security Project - Top 10 risks
The OWASP Mobile Security Project is a centralized resource intended to give developers and security teams the resources they need to build and maintain secure mobile applications. Through the project, our goal is to classify mobile security risks and provide developmental controls to reduce their impact or likelihood of exploitation.
OWASP - Mobile Security Testing Guide:
The Mobile Security Testing Guide (MSTG) is a comprehensive manual for mobile app security development, testing and reverse engineering.
The Web Security Testing Guide:
The OWASP Web Security Testing Guide includes a "best practice" penetration testing framework which users can implement in their own organizations and a "low level" penetration testing guide that describes techniques for testing most common web application and web service security issues.