Logged out sessions are resurrected by concurrent requests

[Jonas Borgström]

1. User logs in
2. User loads a slow page in separate tab or as an ajax request, which modifies the session
3. User logs out before request in step 2 completes. This will delete the session from the db

Expected behavior

User/session stays logged out since the user explicitly logged out and the session row was delete in step 3.

Actual behavior

The previously deleted session is re-inserted into the database when the request from step 2 completes. So the previously logged out user is now logged in again.

[Jonas Borgström]

Proposed fix (against django 1.4)

[Jonas Borgström]

bump

[Russell Keith-Magee]

Seems like a reasonable request, and the patch looks like a decent start -- but it needs tests.

[nikl@…]

Finalized on the train and airport: ​https://github.com/django/django/pull/2678

Thanks to everybody at DjangoIsland who helped me tackle this - looking forward to your feedback!

• Nikl

[Tim Graham]

Please don't mark your own patch as RFC. Someone who reviews the patch should do that.

[Sasha Romijn]

I'm not entirely getting this. When a user logs out, ​the session is flushed. Flushing the session ​clears it and deletes it. The database session store performs this deletion by actually ​deleting the record from the DB. The cached_db backend ​deletes it from the DB and the cache. So basically, all records of this session should be deleted. If you would post a new request with the now deleted session ID, ​Django will reject it, and assign you a new session with a new session ID.

The reporter says that django re-inserts the session when a request arrives with the old session ID, and will re-insert it with the old session data. But I don't see that anywhere in the code. As far as I can see, Django would reject the session ID, as loading would fail as the session object has been deleted, and the user would be assigned a new session. Even if there were a flaw in that logic: once the session data has been deleted, how would any code know how to recreate the session? The request doesn't contain any hint on what user should be logged in.

The only explanation I can come up with is that we're talking about cookie backed sessions, for which this is a documented limitation: you can't guarantee deletion of a cookie backed session or it's data, no matter what we do in Django: its the nature of cookies.

[Tim Graham]

I couldn't reproduce this using steps 1-3 in the description (SQLite). After logging out in a separate tag, the slow page loaded, but subsequent requests redirected to the admin login page. There also seem to be some concerns from Nick's review on the PR.

[Jonas Borgström]

I think one key detail missing from the initial reproduction steps is that the "slow page" needs to modify the session to make it dirty. Otherwise the session will not be resurrected.

Anyway, I've now create a complete reproduction test case here:
​https://github.com/jborg/django-21608

See README.txt for details.

[Collin Anderson]

would session.save(force_update=True) fix this issue?