security - client secret in OAuth 2.0

Question

To use google drive api, I have to play with the authentication using OAuth2.0. And I got a few question about this.

1. Client id and client secret are used to identify what my app is. But they must be hardcoded if it is a client application. So, everyone can decompile my app and extract them from source code. Does it mean that a bad app can pretend to be a good app by using the good app's client id and secret? So user would be showing a screen that asking for granting permission to a good app even though it is actually asked by a bad app? If yes, what should I do? Or actually I should not worry about this?

2. In mobile application, we can embedded a webview to our app. And it is easy to extract the password field in the webview because the app that asking for permission is actually a "browser". So, OAuth in mobile application does not have the benefit that client application has not access to the user credential of service provider?

Answer 1

I had the same question as the question 1 here, and did some research myself recently, and my conclusion is that it is ok to not keep "client secret" a secret. The type of clients that do not keep confidentiality of client secret is called "public client" in the OAuth2 spec. The possibility of someone malicious being able to get authorization code, and then access token, is prevented by the following facts.

1. Client need to get authorization code directly from the user, not from the service

Even if user indicates the service that he/she trusts the client, the client cannot get authorization code from the service just by showing client id and client secret. Instead, the client has to get the authorization code directly from the user. (This is usually done by URL redirection, which I will talk about later.) So, for the malicious client, it is not enough to know client id/secret trusted by the user. It has to somehow involve or spoof user to give it the authorization code, which should be harder than just knowing client id/secret.

2. Redirect URL is registered with client id/secret

Let’s assume that the malicious client somehow managed to involve the user and make her/him click "Authorize this app" button on the service page. This will trigger the URL redirect response from the service to user’s browser with the authorization code with it. Then the authorization code will be sent from user’s browser to the redirect URL, and the client is supposed to be listening at the redirect URL to receive the authorization code. (The redirect URL can be localhost too, and I figured that this is a typical way that a “public client” receives authorization code.) Since this redirect URL is registered at the service with the client id/secret, the malicious client does not have a way to control where the authorization code is given to. This means the malicious client with your client id/secret has another obstacle to obtain the user’s authorization code.