client secret in OAuth 2.0

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.

I started writing a comment to your question but then found out there is too much to say so here are my views on the subject in the answer.

Yes there is a real possibility for this and there were some exploits based on this. Suggestion is not to keep the app secret in your app, there is even part in the spec that distributed apps should not use this token. Now you might ask, but XYZ requires it in order to work. In that case they are not implementing the spec properly and you should A not use that service (not likely) or B try to secure token using some obfuscating methods to make it harder to find or use your server as a proxy. For example there were some bugs in Facebook library for Android where it was leaking tokens to Logs, you can find out more about it here http://attack-secure.com/all-your-facebook-access-tokens-are-belong-to-us and here https://www.youtube.com/watch?v=twyL7Uxe6sk. All in all be extra cautious of your usage of third party libraries (common sense actually but if token hijacking is your big concern add another extra to cautious). I have been ranting about the point 2 for quite some time. I have even done some workarounds in my apps in order to modify the consent pages (for example changing zoom and design to fit the app) but there was nothing stopping me from reading values from fields inside the web view with username and password. Therefore I totally agree with your second point and find it a big "bug" in OAuth spec. Point being "App doesn't get access to users credentials" in the spec is just a dream and gives users false sense of security… Also I guess people are usually suspicions when app asks them for their Facebook, Twitter, Dropbox or other credentials. I doubt many ordinary people read OAuth spec and say "Now I am safe" but instead use common sense and generally not use apps they don't trust.

Answering to 2nd question: Google APIs for security reason mandate that authentication/sign-in cannot be done within App itself (like webviews are not allowed) and needs to be done outside app using Browser for better security which is further explained below: https://developers.googleblog.com/2016/08/modernizing-oauth-interactions-in-native-apps.html