4 ms·

What am I missing here? How do you mess this up? Like you redirect to the OAuth provider with a url that has your client_id and ask for a postback and you get i

by Spivak 3y ago

What am I missing here? How do you mess this up? Like you redirect to the OAuth provider with a url that has your client_id and ask for a postback and you get it with a code, you use that code plus your app's client_id/secret to get user-creds and then query some API to figure out who just authed. The code is tied to your client_id/secret. You have to implicitly verify the code you got just to figure out who logged in.

michaeljx 3y ago
An attacker could call the post back with a code generated for another client_id. When you use the code to query who logged in, Facebook will still respond, even if the code was not generated for your client_id
- rosswilson 3y ago
  This shouldn't be possible as the server-to-server request to Facebook to exchange the Authorization Code for an Access Token requires the client_id and client_secret to be provided. Facebook should (though I haven't actually confirmed this) verify that the code was issued to the given client_id. If the code was issued for client 123, when client 456 tries to exchange it for an Access Token Facebook should throw an error.
  - michaeljx 3y ago
    Yup that's the thing, Facebook doesn't do that. The token is the only thing needed, no validation on their half if it matches a client id
jorams 3y ago
What you describe is the explicit grant flow and the exchange of the code for an access token automatically protects against this vulnerability. The bugs here are in the implicit grant flow where the redirect doesn't include an opaque code but an actual access token. There the application is responsible for validating that the token is for that application, but there's nothing that requires it to do that.
- dboreham 3y ago
  I'm a little confused about this. The article seems to say that the application (the server that is the target of the attack) makes a call to FB where the application believes that a successful response indicates that the user's credentials are valid. It says that FB responds with success (and the user's email). But in fact FB would respond with success regardless of whether the supplied token was valid (for that application). This seems like either a serious bug in FB's implementation, or very bad API design. Presumably it's the latter because FB is responding correctly to a request that has nothing to do with auth -- application (server) asked for the email for one of its users, providing its (the server's) credentials for that request. The problem is that the API doc/design has fooled the application developer into believing that that call was an auth check. But it wasn't. Is that it? Also...why is this not wrapped in a library such that it wouldn't be possible to make such a mistake?
  - locallost 3y ago
    "Facebook" (or whoever) just sends a token that says the user authorized the use of its account for the application. What happens in this 'hack' is that the token is valid, but was not actually intended for that application, but for another one (e.g. a malicious one, that is now trying to reuse the token to access other services of the same Facebook user). I don't know how Facebook tokens look like, but JWTs have a property called aud/audience indicating who the token was intended for. The application developer must verify that the token was actually intended for their application. That is my understanding, sorry if I got it wrong.
    - dboreham 3y ago
      I get that. You're not wrong. I'm asking why Facebook tells the application anything other than "nope, that's not valid for you"? It doesn't seem to be a case of the application will accept random noise as a valid token. It does send the token to Facebook and Facebook responds with something that looks like "OK". But when they said "OK" they really meant "we gave you some valid information about the target user, but forgot to mention that the token you provided was not valid". I'm saying that Facebook should in all cases respond with "Nope, that's not a valid token". Their API just shouldn't have a way to get data on a user without providing a valid token (by valid, we mean valid for that purpose, not stolen from somewhere).

by Spivak 3y ago

michaeljx 3y ago

An attacker could call the post back with a code generated for another client_id. When you use the code to query who logged in, Facebook will still respond, even if the code was not generated for your client_id

rosswilson 3y ago
This shouldn't be possible as the server-to-server request to Facebook to exchange the Authorization Code for an Access Token requires the client_id and client_secret to be provided. Facebook should (though I haven't actually confirmed this) verify that the code was issued to the given client_id. If the code was issued for client 123, when client 456 tries to exchange it for an Access Token Facebook should throw an error.
- michaeljx 3y ago
  Yup that's the thing, Facebook doesn't do that. The token is the only thing needed, no validation on their half if it matches a client id

jorams 3y ago

What you describe is the explicit grant flow and the exchange of the code for an access token automatically protects against this vulnerability. The bugs here are in the implicit grant flow where the redirect doesn't include an opaque code but an actual access token. There the application is responsible for validating that the token is for that application, but there's nothing that requires it to do that.

dboreham 3y ago
I'm a little confused about this. The article seems to say that the application (the server that is the target of the attack) makes a call to FB where the application believes that a successful response indicates that the user's credentials are valid. It says that FB responds with success (and the user's email). But in fact FB would respond with success regardless of whether the supplied token was valid (for that application). This seems like either a serious bug in FB's implementation, or very bad API design. Presumably it's the latter because FB is responding correctly to a request that has nothing to do with auth -- application (server) asked for the email for one of its users, providing its (the server's) credentials for that request. The problem is that the API doc/design has fooled the application developer into believing that that call was an auth check. But it wasn't. Is that it? Also...why is this not wrapped in a library such that it wouldn't be possible to make such a mistake?
- locallost 3y ago
  "Facebook" (or whoever) just sends a token that says the user authorized the use of its account for the application. What happens in this 'hack' is that the token is valid, but was not actually intended for that application, but for another one (e.g. a malicious one, that is now trying to reuse the token to access other services of the same Facebook user). I don't know how Facebook tokens look like, but JWTs have a property called aud/audience indicating who the token was intended for. The application developer must verify that the token was actually intended for their application. That is my understanding, sorry if I got it wrong.
  - dboreham 3y ago
    I get that. You're not wrong. I'm asking why Facebook tells the application anything other than "nope, that's not valid for you"? It doesn't seem to be a case of the application will accept random noise as a valid token. It does send the token to Facebook and Facebook responds with something that looks like "OK". But when they said "OK" they really meant "we gave you some valid information about the target user, but forgot to mention that the token you provided was not valid". I'm saying that Facebook should in all cases respond with "Nope, that's not a valid token". Their API just shouldn't have a way to get data on a user without providing a valid token (by valid, we mean valid for that purpose, not stolen from somewhere).