Client secret + refreshing the access token in spring oauth2

Question

I am using spring boot for backend and Android device for frontend of my system. Right now I am facing the challenge to use Spring-OAuth2 to secure my resource server.
I have some questions, which I want to discuss with you:
My knowledge + this tutorial are saying that I should use the OAuth2.0 "password" grant type for my mobile app to obtain an access token. The official spring tutorial for security gives an example how to obtain the access token using password grant type:

$ curl client:secret@localhost:8080/oauth/token -d grant_type=password -d username=user -d password=pwd

And here comes my first question: Is there any possibility to obtain access token using the password grant type without sending the "client secret" ?
Since the client secret could be "reverse engineered" by decompiling the client app. The obtaining access token without secret should be somehow possible, because Facebook SDK for Android also does not need the client_secret in the mobile app.
I think here I have a little trouble understanding why the clientID + clientSecret needs to be included in the request above, because, since there are already username + password included, it should be possible to generate the access token, so does this brings a next level of security ? and does it implies the following (example): I am logged in as Filip in my Android client and I am sending the access token A with each request to the server. Then I log in as Filip into web client and I try to access the resource server from web client using the access token A, which is not possible because access token A was issued only for Android client ?

The next question is how can I refresh the obtained access token ?
I was trying to do so using the command below, but I got "Full authentication is required to access this resource." After I got the new refreshed token, can I use the refresh token to refresh my new access token again ?

curl -v --data "grant_type=refresh_token&client_id=acme&client_secret=acmesecret&refresh_token=REFRESH_TOKEN" http://localhost:9999/uaa/oauth/token

Thank you

Answer 1

The OAuth 2.0 spec allows for so-called public clients i.e. clients that don't authenticate themselves. So it is possible to use the Resource Owner Password Credentials grant with a public client, i.e. one that does not need to send a client secret. It does mean that the Authorization Server cannot assume anything about the client since a client_id is not a secret and there's no way to prevent a malicious client using this grant type or clients from impersonating each other. So using it in this way comes at the cost of reduced security although one may argue that in your case there's no way to use confidential clients anyhow, so there's no difference.

In general the Resource Owner Password Credentials grant is an anti-pattern for OAuth and only meant for migration purposes because it defeats most of the goals of OAuth in itself.

Access tokens are issued on a per-client basis.

You refresh token request seems OK but the Authorization Server may require basic authentication instead of providing the client_id/client_secret as post parameters, considering that you did the same for the original access token request.