Form wizard may raise unreasonable exceptions in case of SECRET_KEY change

[Sasha Romijn]

Our form wizard has two storage options: sessions and cookies, with SessionWizardView and CookieWizardView. To prevent manipulation, the cookies storage uses the signed cookies from django.core.signing. This creates a signature based on the SECRET_KEY. If the secret key is changed, request.get_signed_cookie will raise an exception, in which case the storage will raise WizardViewCookieModified, a subclass of SuspiciousOperation.

The cookie is loaded very early in the rendering of a the form wizard view. This means that if a user starts a form wizard, and the secret key is changed, any requests to the wizard will result in an exception and likely a 500 error. The user can only recover from this by deleting the cookie or restarting the browser (it seems to only persist in the current session).

It may appear sensible to raise a SuspiciousOperation for a possible cookie manipulation, but we currently don't do this in any other place, like sessions. Currently, user may suddenly get 500 errors for no clear reason, and the developer of the project has no ability to help them. Leaving this as is may also discourage people from rotating their secret key when needed.

I suggest that in case of an invalid wizard cookie, we simply ignore the value and thereby return the user to the first step.

[nott]

WizardViewCookieModified, a subclass of SuspiciousOperation, is raised in this situation since 1.6

[Sasha Romijn]

Apologies, forgot to assign this to myself before working on it. Wrote a PR on ​https://github.com/django/django/pull/2673

[Denis Cornehl]

tested the behavior with a test-application, looks fine to me.

[Sasha Romijn]

Existing PR updated: ​​https://github.com/django/django/pull/2673

[Erik Romijn <eromijn@…>]

In ba5ddf7aed542d25f0fdb25a04d87305de0f3972:

Fixed #22638 -- Changed CookieWizardView to ignore invalid cookies