Manually logging in a user without password

You don’t need a password to log a user in. The auth.login function just takes a User object, which you are presumably already getting from the database when you enable the account. So you can pass that straight to login.

Of course, you’ll need to be very careful that there’s no way a user can spoof a link to an existing already-enabled account, which would then automatically log them in as that user.

from django.contrib.auth import login

def activate_account(request, hash):
    account = get_account_from_hash(hash)
    if not account.is_active:
        account.activate()
        account.save()
        user = account.user
        login(request, user)

… etc.

Edited:

Hmm, didn’t notice that requirement to use authenticate because of the extra property it adds. Looking at the code, all it does is a backend attribute equivalent to the module path of the authenticating backend. So you could just fake it – before the login call above, do this:

user.backend = 'django.contrib.auth.backends.ModelBackend'