Allow storing CSRF tokens in sessions

[Paul McMillan]

Storing the CSRF token in a cookie (Django's default) is safe, but storing it in the session is common practice in other web frameworks and therefore sometimes demanded by security auditors.

[Raphael Michel]

In case other people at the #duth sprint are looking into this: Shai Berger is working on a new approach to generating the tokens and I'm working on the usage of sessions for token storage.

[Raphael Michel]

I submitted a first version of a patch for session storage of CSRF tokens: ​https://github.com/django/django/pull/5600

I'd love to have some review on this, but I'm fine with postponing the merge after Shai landed his changes to CSRF handling, as those two will get merge conflicts and his one will be the bigger change.

[Tim Graham <timograham@…>]

In ddf169c:

Refs #16859 -- Allowed storing CSRF tokens in sessions.

Major thanks to Shai for helping to refactor the tests, and to
Shai, Tim, Florian, and others for extensive and helpful review.

[Tim Graham <timograham@…>]

In 33e86b34:

Refs #16859 -- Disabled CSRF_COOKIE_* checks when using CSRF_USE_SESSIONS.

[Tim Graham <timograham@…>]

In 503e944a:

Refs #16859 -- Updated CSRF FAQ to mention CSRF_USE_SESSIONS setting.

[Tim Graham <timograham@…>]

In 6bb01b0:

[1.11.x] Refs #16859 -- Updated CSRF FAQ to mention CSRF_USE_SESSIONS setting.

Backport of 503e944ac792498e7b38c799d8e4b06f74e9d65a from master

[Ed Morley]

Currently the CSRF middleware performs strict Referer header checking, to (a) mitigate MITM attacks that set a cookie via plain HTTP, and (b) prevent issues with malicious subdomains.

If the new CSRF_USE_SESSIONS is set to True, does that mean both of those issues can no longer occur, and so the strict referrer checking is then not required? (Along the lines of: ​https://github.com/django/django/pull/5600#issuecomment-154797097)

[Tim Graham]

I'm going to retitle this ticket for the work that was done and close it.

If there's remaining work from items in the old description (below), let's open new tickets to track them individually.

This is a ticket to keep track of general CSRF improvements we want to add to Django. This includes:

• #16010 - add Origin checking
• Optionally tie CSRF to sessions [done in this ticket]
• Use signing to improve CSRF (maybe with sessions)
• Improve domain/host checking - deal with the subdomain to subdomain problem