Invalidate an old session in Flask


Question

How do I create a new clean session and invalidate the current one in Flask?

Do I use make_null_session() or open_session()?

1
22
3/21/2013 6:53:36 PM

Accepted Answer

I do this by calling session.clear().

EDIT:

After reading your comment in another answer, I see that you're trying to prevent a replay attack that might be made using a cookie that was issued in the past. I solved that problem as much as possible* with this approach:

  • Override SecureCookieSessionInterface.save_session(), copying the code from the overridden version rather than calling it.
  • When the overridden version of save_session() calls save_cookie(), make it pass a session_expires argument 30 minutes in the future. This causes cookies more than 30 minutes old to be considered invalid.
  • Make the overridden version of save_session() update a session variable every so often, to make sure the cookie and its session_expires time get rewritten regularly. (I name this session variable '_refresh' and store the current time in it, then rewrite it only if more than a few seconds have passed since the last-stored time. This optimization avoids rewriting the cookie on every HTTP request.)

Duplicating Flask code in the custom save_session() makes this approach a bit ugly and brittle, but it is necessary in order to change the arguments passed to save_cookie(). It would be nice if Flask made this easier, or at least implemented its own safeguard against replay attacks.

*WARNING: This approach by itself will not stop replay attacks that might happen during a session cookie's valid lifetime. This fundamental problem with cookie-based sessions is discussed in RFC 6896 and A Secure Cookie Protocol by Liu, Kovacs, Huang, Gouda.

26
10/13/2016 6:25:57 AM

If you have security concerns (and everyone should have) There is the answer:

This is not REALLY possible

Flask uses cookie-based sessions. When you edit or delete session, you send a REQUEST to CLIENT to remove the cookie, normal clients (browsers) will do. But if session hijacked by an attacker, the attacker's session remains valid.


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