Re: [OAUTH-WG] Why give the redirect URI when trading an [authorization] code for an access token?

["Freeman, Tim" <[e-mail]>]

From: Eran Hammer-Lahav [mailto:[e-mail]] 
> 1. Evil user starts the OAuth flow on the client using the web-server flow.
> 2. Client redirects the evil user to the authorization server, including state
> information about the evil user account on the client.
> 3. Evil user takes the authorization endpoint URI and changes the
> redirection to its own site.
> 4. Evil user tricks victim user to click on the link and authorize access
> (phishing or other social engineering attack).
> 5. Victim user thinking this is a valid authorization request, authorizes
> access.
> 6. Authorization server sends victim user back to the client, but since the
> redirection URI was changed, back to the evil user site.

and the rest of the steps were revised to:

> 7. Evil user takes the code and gives it back to the client by 
> constructing the original correct redirection URI.
> 8. Client exchanges the code for access token, attaching it to the evil user's account.
> 9. Evil user can now access victim user data on his client account.

Step 6 requires the authorization server to redirect the victim user's browser to the evil user's site, which will only happen if the authorization server does not check the redirect URI when returning an authorization code.  That check is required by OAuth 2, and isn't the check that I'm questioning.

In contrast, I was proposing that the authorization server not check the redirect URI when it returns an access token.  Since, in the absence of such a check, the redirect URI is neither checked nor used to compute the result of the query, I was proposing that we drop the redirect URL from that step of the protocol altogether.  

I misspoke in the subject line of the email that started this thread, so I just now changed "access code" there to "authorization code".  We have access tokens and authorization codes, but not authorization tokens or access codes.

After some thought, I think I see the real problem with omitting the check on redirect_uri.   Without the check, the following can happen:

1. Evil user starts OAuth flow on the client using the web-server flow, using a hacked web browser.
2. The hacked web browser does the normal thing in the flow, requesting the evil user's username and password, and at the end the hacked browser is given a redirect like:
   
   https://www.goodclient.com/oauth/evilUserAcct?code=evilAuthCode

The hacked browser saves evilAuthCode and does not perform the redirect.
3. Suppose the evil user's web site can persuade a known victim user with a normal browser to visit goodclient's web site and do OAuth.  The victim's user id on the client is public knowledge, and the evil user's web site persuaded the victim to start OAuth at a known time, so it can guess when the victim will complete the OAuth exchange.  Before then, the evil web site can visit

   https://www.goodclient.com/oauth/victimUserAcct?code=evilAuthCode

which is likely to leave the client associating the victim's user id with an access token that accesses resources controlled by the evil user. 

Checking the redirect URI doesn't solve the problem in the case where the user id on the client is encoded in the "state" opaque value instead of the redirect URI.

Saying "The authorization server SHOULD require the client to pre-register their redirection URI" (page 17 of http://tools.ietf.org/html/draft-ietf-oauth-v2-10) should have MUST instead of SHOULD, since it is important that we're really redirecting to the client when we're distributing the authorization code by redirecting.
 
Writing the spec required thinking through these cases.  It would be helpful if the use cases were added to the spec, so people don't have to rediscover them, and errors in the use cases can be discovered and fixed rather than being made repeatedly by each person rediscovering the use case.

Tim Freeman
Email: [e-mail]
Desk in Palo Alto: [phone]
Home: [phone]
Cell: [phone]


-----Original Message-----
From: Eran Hammer-Lahav [mailto:[e-mail]] 
Sent: Saturday, September 11, 2010 7:59 AM
To: Torsten Lodderstedt
Cc: Freeman, Tim; [e-mail]
Subject: RE: [OAUTH-WG] Why give the redirect URI when trading an access code for an access token?

Sorry.

7. Evil user takes the code and gives it back to the client by constructing the original correct redirection URI.
8. Client exchanges the code for access token, attaching it to the evil user's account.
9. Evil user can now access victim user data on his client account.

This is basically a session fixation attack.

EHL

> -----Original Message-----
> From: Torsten Lodderstedt [mailto:[e-mail]]
> Sent: Saturday, September 11, 2010 1:01 AM
> To: Eran Hammer-Lahav
> Cc: Freeman, Tim; [e-mail]
> Subject: Re: [OAUTH-WG] Why give the redirect URI when trading an access
> code for an access token?
> 
>   Doesn't step 7 require the evil user to know the client's secret?
> 
> Am 10.09.2010 17:06, schrieb Eran Hammer-Lahav:
> > 1. Evil user starts the OAuth flow on the client using the web-server flow.
> > 2. Client redirects the evil user to the authorization server, including state
> information about the evil user account on the client.
> > 3. Evil user takes the authorization endpoint URI and changes the
> redirection to its own site.
> > 4. Evil user tricks victim user to click on the link and authorize access
> (phishing or other social engineering attack).
> > 5. Victim user thinking this is a valid authorization request, authorizes
> access.
> > 6. Authorization server sends victim user back to the client, but since the
> redirection URI was changed, back to the evil user site.
> > 7. Evil user grabs the code and exchanges it for an access token.
> >
> > By checking that the callback URI used to deliver the code is the same as
> the one used to initiate the flow, the authorization server can verify that the
> user who initiated the flow is the same one to authorize access and finish the
> flow.
> >
> > EHL
> >
> >> -----Original Message-----
> >> From: [e-mail] [mailto:[e-mail]] On
> >> Behalf Of Freeman, Tim
> >> Sent: Wednesday, September 08, 2010 8:05 PM
> >> To: [e-mail]
> >> Subject: [OAUTH-WG] Why give the redirect URI when trading an access
> >> code for an access token?
> >>
> >> Hi.  I'm new here.  I searched the archives a bit and didn't
> >> immediately find an answer to my question below.  My apologies if
> >> there was some previous discussion of this that I missed.
> >>
> >> Looking at the draft spec at
> >> http://tools.ietf.org/html/draft-ietf-oauth-v2-10,
> >> I see in section 4.1.1 "Authorization code" on page 22 that it is
> >> required to give the redirect_uri of the original request when
> >> exchanging an authorization code for an access token, and the
> >> authorization server must verify that the redirection URI is correct as well
> as the authorization code.
> >> Based on section 4.2 "Access Token Response" on page 25, it seems
> >> that the redirect_uri is not used when constructing the response from
> >> the authorization server.
> >>
> >> So far as I can tell, the redirect_uri is useless in this request.
> >> It does not contain any secrets.  The authorization code is verified
> >> and is meant to be an arbitrary unguessable identifier, so little is
> >> gained by verifying the redirect_uri also.  It is not used to construct the
> reply.  Why is it required?
> >>
> >> Tim Freeman
> >> Email: [e-mail]
> >> Desk in Palo Alto: [phone]
> >> Home: [phone]
> >> Cell: [phone]
> >>
> >>
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