Managing users authentication in Google App Engine

Question

I am working on a webapp based on google app engine. The application uses the google authentication apis. Basically every handler extends from this BaseHandler and as first operation of any get/post the checkAuth is executed.

class BaseHandler(webapp2.RequestHandler):
googleUser = None
userId = None
def checkAuth(self):
    user = users.get_current_user()
    self.googleUser = user;
    if user:
        self.userId = user.user_id()
        userKey=ndb.Key(PROJECTNAME, 'rootParent', 'Utente', self.userId)
        dbuser = MyUser.query(MyUser.key==userKey).get(keys_only=True)
        if dbuser:
            pass
        else:
            self.redirect('/')
    else:
        self.redirect('/')

The idea is that it redirects to / if no user is logged in via Google OR if there is not a User in my db of users having that google id.

The problem is that I can succesfully log in my web app and make operations. Then, from gmail, o Logout from any google account BUT if i try to keep using the web app it works. This means the users.get_current_user() still returns a valid user (valid but actually OLD). Is that possible?

IMPORTANT UPDATE I Do Understand what explained in the Alex Martelli's Comment: There is a cookie which keeps the former GAE authentication valid. The problem is that the same web app also exploits the Google Api Client Library for Python https://developers.google.com/api-client-library/python/ to perform operations on Drive and Calendar. In GAE apps such library can be easily used through decorators implementing the whole OAuth2 Flow (https://developers.google.com/api-client-library/python/guide/google_app_engine).

I therefore have my Handlers get/post methods decorated with oauth_required like this

class SomeHandler(BaseHandler):
    @DECORATOR.oauth_required
    def get(self):
        super(SomeHandler,self).checkAuth()
        uid = self.googleUser.user_id()
        http = DECORATOR.http()
        service = build('calendar', 'v3')
        calendar_list = service.calendarList().list(pageToken=page_token).execute(http=http)

Where decorator is

   from oauth2client.appengine import OAuth2Decorator

   DECORATOR = OAuth2Decorator(
  client_id='XXXXXX.apps.googleusercontent.com',
  client_secret='YYYYYYY',
  scope='https://www.googleapis.com/auth/calendar https://www.googleapis.com/auth/drive https://www.googleapis.com/auth/drive.appdata https://www.googleapis.com/auth/drive.file'

        )

It usually works fine. However (!!) when the app is idle for a long time it happens that the oauth2 decorator redirects me to the Google authentication page where, if I change account (I have 2 different accounts) Something WEIRD happens: The app is still logged as the former account (retrieved through users.get_current_user()) while the api client library, and thus the oauth2 decorator, returns data (drive, calendar, etc.) belonging to the second account.

Which is REALLY not appropriate.

Following the example above (SomeHandler class) suppose I am logged as Account A. The users.get_current_user() always returns A as expected. Now suppose I stopped using the app, after a long while the oauth_required redirects me to the Google Account page. I therefore decide (or make a mistake) to log is as Account B. When accessing the Get method of the SomeHandler class the userId (retrived through users.get_current_user() is A while the list of calendars returned through the service object (Google Api client Library) is the list of calendars belonging to B (the actual currently logged user).

Am I doing something wrong? is Something expected?

Another Update

this is after the Martelli's Answer. I have updated the handlers like this:

class SomeHandler(BaseHandler):
    @DECORATOR.oauth_aware
    def get(self):
        if DECORATOR.has_credentials():
            super(SomeHandler,self).checkAuth()
            uid = self.googleUser.user_id()
            try:
               http = DECORATOR.http()
               service = build('calendar', 'v3')
               calendar_list = service.calendarList().list(pageToken=page_token).execute(http=http)
            except (AccessTokenRefreshError, appengine.InvalidXsrfTokenError):
                self.redirect(users.create_logout_url(
                    DECORATOR.authorize_url()))
        else:
           self.redirect(users.create_logout_url(
              DECORATOR.authorize_url()))

so basically I now use oauth_aware and, in case of none credentials I logout the user and redirect it to the DECORATOR.authorize_url()

I have noticed that after a period of inactivity, the handler raises AccessTokenRefreshError and appengine.InvalidXsrfTokenError exceptions (but the has_credentials() method returns True). I catch them and (again) redirect the flow to the logout and authorize_url()

It seems to work and seems to be robust to accounts switch. Is it a reasonable solution or am I not considering some aspects of the issue?

Sure it's possible -- the user's browser still has the cookies to authenticate to GAE. To log the user out of your app specifically, see cloud.google.com/appengine/docs/python/users/… . — Alex Martelli
– Alex Martelli, Commented Feb 22, 2015 at 19:00
Hi Alex, thank you for your kind help. I have updated my question and I do hope you will be so kind to help me again — lowcoupling
– lowcoupling, Commented Feb 23, 2015 at 11:42
Having some trouble reproducing the issue, can you please post a minimal misbehaving case as explained at stackoverflow.com/help/mcve ? — Alex Martelli
– Alex Martelli, Commented Feb 23, 2015 at 14:51
Hi Alex I have tried to add some details hoping it is enough to highlight some major mistake I am doing. The behavior I am experiencing is so weird that I assume there has to be something wrong in my implementation — lowcoupling
– lowcoupling, Commented Feb 23, 2015 at 15:41

Alex Martelli · Accepted Answer · 2015-02-23 23:25:50Z

I understand the confusion, but the system is "working as designed".

At any point in time a GAE handler can have zero or one "logged-in user" (the object returned by users.get_current_user(), or None if no logged-in user) and zero or more "oauth2 authorization tokens" (for whatever users and scopes have been granted and not revoked).

There is no constraint that forces the oauth2 thingies to match, in any sense, the "logged-in user, if any".

I would recommend checking out the very simple sample at https://code.google.com/p/google-api-python-client/source/browse/samples/appengine/main.py (to run it, you'll have to clone the whole "google-api-python-client" package, then copy into the google-api-python-client/source/browse/samples/appengine directory directories apiclient/ and oauth2client/ from this same package as well as httplib2 from https://github.com/jcgregorio/httplib2 -- and also customize the client_secrets.json -- however, you don't need to run it, just to read and follow the code).

This sample doesn't even use users.get_current_user() -- it doesn't need it nor care about it: it only shows how to use oauth2, and there is no connection between holding an oauth2-authorized token, and the users service. (This allows you for example to have cron execute on behalf of one or more users certain tasks later -- cron doesn't log in, but it doesn't matter -- if the oauth2 tokens are properly stored and retrieved then it can use them).

So the code makes a decorator from the client secrets, with scope='https://www.googleapis.com/auth/plus.me', then uses @decorator.oauth_required on a handler's get to ensure authorization, and with the decorator's authorized http, it fetches

user = service.people().get(userId='me').execute(http=http)

with service built earlier as discovery.build("plus", "v1", http=http) (with a different non-authorized http).

Should you run this locally, it's easy to add a fake login (remember, user login is faked with dev_appserver) so that users.get_current_user() returns [email protected] or whatever other fake email you input at the fake login screen -- and this in no way inhibits the completely separate oauth2 flow from still performing as intended (i.e, exactly the same way as it does without any such fake login).

If you deploy the modified app (with an extra user login) to production, the login will have to be a real one -- but it's just as indifferent to, and separate from, the oauth2 part of the app.

If your application's logic does require constraining the oauth2 token to the specific user who's also logged into your app, you'll have to implement this yourself -- e.g by setting scope to 'https://www.googleapis.com/auth/plus.login https://www.googleapis.com/auth/plus.profile.emails.read' (plus whatever else you need), you'll get from service.people().get(userId='me') a user object with (among many other things) an emails attribute in which you can check that the authorization token is for the user with the email you intended to authorize (and take remedial action otherwise, e.g via a logout URL &c). ((This can be done more simply and in any case I doubt you really need such functionality, but, just wanted to mention it)).

Thank you very much for your kind help. Retrieving the user from G+ would consume quotas while i just need to check whether the token is related to a specific userid, is there any quota free method to do so ?
@lowcoupling, it seems to me your another update edit is roughly the best you can do.

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