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'

As of Django 1.10, the process has been simplified.

In all versions of Django, in order for a user to be logged in, they must be authenticated by one of your app's backends (controlled by the AUTHENTICATION_BACKENDS setting).

If you simply want to force a login, you can just claim that the user was authenticated by the first backend from that list:

from django.conf import settings
from django.contrib.auth import login


# Django 1.10+
login(request, user, backend=settings.AUTHENTICATION_BACKENDS[0])

# Django <1.10 -  fake the authenticate() call
user.backend = settings.AUTHENTICATION_BACKENDS[0]
login(request, user)

Daniel's answer is very good.

Another way to do it is to create a HashModelBackend following the Custom Authorization backends https://docs.djangoproject.com/en/1.8/topics/auth/customizing/#writing-an-authentication-backend like this:

class HashModelBackend(object):
    def authenticate(self, hash=None):
        user = get_user_from_hash(hash)
        return user

    def get_user(self, user_id):
        try:
            return User.objects.get(pk=user_id)
        except User.DoesNotExist:
            return None

And then install this in your settings:

AUTHENTICATION_BACKENDS = (
    'myproject.backends.HashModelBackend',
    'django.contrib.auth.backends.ModelBackend',
)

Then your view would be something like this:

def activate_account(request, hash):
    user = authenticate(hash=hash)
    if user:
        # check if user is_active, and any other checks
        login(request, user)
    else:
        return user_not_found_bad_hash_message

Response to dan's answer.

A way to write your backend:

from django.contrib.auth import get_user_model
from django.contrib.auth.backends import ModelBackend

class HashModelBackend(ModelBackend):

def authenticate(self, username=None, **kwargs):
    UserModel = get_user_model()
    if username is None:
        username = kwargs.get(UserModel.USERNAME_FIELD)
    try:
        user = UserModel._default_manager.get_by_natural_key(username)
        return user
    except UserModel.DoesNotExist:
        return None

Answer is based on django.contrib.auth.backends.ModelBackend source code. It's actual for django 1.9

And I would rather place custom backend below django's default:

AUTHENTICATION_BACKENDS = [
    'django.contrib.auth.backends.ModelBackend',
    'yours.HashModelBackend',
]

because account activation is less possible than login itself. According to https://docs.djangoproject.com/en/1.9/topics/auth/customizing/#specifying-authentication-backends:

The order of AUTHENTICATION_BACKENDS matters, so if the same username and password is valid in multiple backends, Django will stop processing at the first positive match.

Be careful this code will authenticate your users even with incorrect passwords.

You can use ska package, which has password-less login to Django implemented. ska works with authentication tokens and its security is based on SHARED_KEY which should be equal for all parties (servers) involved.

On client side (party that requests a password-less login), you generate a URL and sign it, using ska. Example:

from ska import sign_url
from ska.contrib.django.ska.settings import SECRET_KEY

server_ska_login_url = 'https://server-url.com/ska/login/'

signed_url = sign_url(
    auth_user='test_ska_user_0',
    secret_key=SECRET_KEY,
    url=server_ska_login_url
    extra={
        'email': '[email protected]',
        'first_name': 'John',
        'last_name': 'Doe',
    }
)

Default lifetime of the token is 600 seconds. You can customise that by proving a lifetime argument.

On the server side (site to which users' log in), having in mind that you have installed ska properly, the user is logged in upon visiting the URL if they existed (username match), or otherwise - created. There are 3 callbacks that you can customise in your project's Django settings.

USER_GET_CALLBACK (string): Fired if user was successfully fetched from database (existing user).
USER_CREATE_CALLBACK (string): Fired right after user has been created (user didn't exist).
USER_INFO_CALLBACK (string): Fired upon successful authentication.

See the documentation (http://pythonhosted.org/ska/) for more.

Manually logging in a user without password

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