How to use sessions¶
Django provides full support for anonymous sessions. The session framework lets you store and retrieve arbitrary data on a per-site-visitor basis. It stores data on the server side and abstracts the sending and receiving of cookies. Cookies contain a session ID – not the data itself (unless you’re using the cookie based backend).
Enabling sessions¶
Sessions are implemented via a piece of middleware.
To enable session functionality, do the following:
- Edit the
MIDDLEWARE
setting and make sure it contains'django.contrib.sessions.middleware.SessionMiddleware'
. The defaultsettings.py
created bydjango-admin startproject
hasSessionMiddleware
activated.
If you don’t want to use sessions, you might as well remove the
SessionMiddleware
line from MIDDLEWARE
and
'django.contrib.sessions'
from your INSTALLED_APPS
.
It’ll save you a small bit of overhead.
Configuring the session engine¶
By default, Django stores sessions in your database (using the model
django.contrib.sessions.models.Session
). Though this is convenient, in
some setups it’s faster to store session data elsewhere, so Django can be
configured to store session data on your filesystem or in your cache.
Using database-backed sessions¶
If you want to use a database-backed session, you need to add
'django.contrib.sessions'
to your INSTALLED_APPS
setting.
Once you have configured your installation, run manage.py migrate
to install the single database table that stores session data.
Using cached sessions¶
For better performance, you may want to use a cache-based session backend.
To store session data using Django’s cache system, you’ll first need to make sure you’ve configured your cache; see the cache documentation for details.
Avertissement
You should only use cache-based sessions if you’re using the Memcached cache backend. The local-memory cache backend doesn’t retain data long enough to be a good choice, and it’ll be faster to use file or database sessions directly instead of sending everything through the file or database cache backends. Additionally, the local-memory cache backend is NOT multi-process safe, therefore probably not a good choice for production environments.
If you have multiple caches defined in CACHES
, Django will use the
default cache. To use another cache, set SESSION_CACHE_ALIAS
to the
name of that cache.
Once your cache is configured, you’ve got two choices for how to store data in the cache:
- Set
SESSION_ENGINE
to"django.contrib.sessions.backends.cache"
for a simple caching session store. Session data will be stored directly in your cache. However, session data may not be persistent: cached data can be evicted if the cache fills up or if the cache server is restarted. - For persistent, cached data, set
SESSION_ENGINE
to"django.contrib.sessions.backends.cached_db"
. This uses a write-through cache – every write to the cache will also be written to the database. Session reads only use the database if the data is not already in the cache.
Both session stores are quite fast, but the simple cache is faster because it
disregards persistence. In most cases, the cached_db
backend will be fast
enough, but if you need that last bit of performance, and are willing to let
session data be expunged from time to time, the cache
backend is for you.
If you use the cached_db
session backend, you also need to follow the
configuration instructions for the using database-backed sessions.
Using file-based sessions¶
To use file-based sessions, set the SESSION_ENGINE
setting to
"django.contrib.sessions.backends.file"
.
You might also want to set the SESSION_FILE_PATH
setting (which
defaults to output from tempfile.gettempdir()
, most likely /tmp
) to
control where Django stores session files. Be sure to check that your Web
server has permissions to read and write to this location.
Using cookie-based sessions¶
To use cookies-based sessions, set the SESSION_ENGINE
setting to
"django.contrib.sessions.backends.signed_cookies"
. The session data will be
stored using Django’s tools for cryptographic signing
and the SECRET_KEY
setting.
Note
It’s recommended to leave the SESSION_COOKIE_HTTPONLY
setting
on True
to prevent access to the stored data from JavaScript.
Avertissement
If the SECRET_KEY is not kept secret and you are using the
PickleSerializer
, this can
lead to arbitrary remote code execution.
An attacker in possession of the SECRET_KEY
can not only
generate falsified session data, which your site will trust, but also
remotely execute arbitrary code, as the data is serialized using pickle.
If you use cookie-based sessions, pay extra care that your secret key is always kept completely secret, for any system which might be remotely accessible.
The session data is signed but not encrypted
When using the cookies backend the session data can be read by the client.
A MAC (Message Authentication Code) is used to protect the data against changes by the client, so that the session data will be invalidated when being tampered with. The same invalidation happens if the client storing the cookie (e.g. your user’s browser) can’t store all of the session cookie and drops data. Even though Django compresses the data, it’s still entirely possible to exceed the common limit of 4096 bytes per cookie.
No freshness guarantee
Note also that while the MAC can guarantee the authenticity of the data
(that it was generated by your site, and not someone else), and the
integrity of the data (that it is all there and correct), it cannot
guarantee freshness i.e. that you are being sent back the last thing you
sent to the client. This means that for some uses of session data, the
cookie backend might open you up to replay attacks. Unlike other session
backends which keep a server-side record of each session and invalidate it
when a user logs out, cookie-based sessions are not invalidated when a user
logs out. Thus if an attacker steals a user’s cookie, they can use that
cookie to login as that user even if the user logs out. Cookies will only
be detected as “stale” if they are older than your
SESSION_COOKIE_AGE
.
Performance
Finally, the size of a cookie can have an impact on the speed of your site.
Using sessions in views¶
When SessionMiddleware
is activated, each HttpRequest
object – the first argument to any Django view function – will have a
session
attribute, which is a dictionary-like object.
You can read it and write to request.session
at any point in your view.
You can edit it multiple times.
-
class
backends.base.
SessionBase
¶ This is the base class for all session objects. It has the following standard dictionary methods:
-
__getitem__
(key)¶ Example:
fav_color = request.session['fav_color']
-
__setitem__
(key, value)¶ Example:
request.session['fav_color'] = 'blue'
-
__delitem__
(key)¶ Example:
del request.session['fav_color']
. This raisesKeyError
if the givenkey
isn’t already in the session.
-
__contains__
(key)¶ Example:
'fav_color' in request.session
-
get
(key, default=None)¶ Example:
fav_color = request.session.get('fav_color', 'red')
-
pop
(key, default=__not_given)¶ Example:
fav_color = request.session.pop('fav_color', 'blue')
-
keys
()¶
-
items
()¶
-
setdefault
()¶
-
clear
()¶
It also has these methods:
-
flush
()¶ Deletes the current session data from the session and deletes the session cookie. This is used if you want to ensure that the previous session data can’t be accessed again from the user’s browser (for example, the
django.contrib.auth.logout()
function calls it).
Sets a test cookie to determine whether the user’s browser supports cookies. Due to the way cookies work, you won’t be able to test this until the user’s next page request. See Setting test cookies below for more information.
Returns either
True
orFalse
, depending on whether the user’s browser accepted the test cookie. Due to the way cookies work, you’ll have to callset_test_cookie()
on a previous, separate page request. See Setting test cookies below for more information.
Deletes the test cookie. Use this to clean up after yourself.
-
set_expiry
(value)¶ Sets the expiration time for the session. You can pass a number of different values:
- If
value
is an integer, the session will expire after that many seconds of inactivity. For example, callingrequest.session.set_expiry(300)
would make the session expire in 5 minutes. - If
value
is adatetime
ortimedelta
object, the session will expire at that specific date/time. Note thatdatetime
andtimedelta
values are only serializable if you are using thePickleSerializer
. - If
value
is0
, the user’s session cookie will expire when the user’s Web browser is closed. - If
value
isNone
, the session reverts to using the global session expiry policy.
Reading a session is not considered activity for expiration purposes. Session expiration is computed from the last time the session was modified.
- If
-
get_expiry_age
()¶ Returns the number of seconds until this session expires. For sessions with no custom expiration (or those set to expire at browser close), this will equal
SESSION_COOKIE_AGE
.This function accepts two optional keyword arguments:
modification
: last modification of the session, as adatetime
object. Defaults to the current time.expiry
: expiry information for the session, as adatetime
object, anint
(in seconds), orNone
. Defaults to the value stored in the session byset_expiry()
, if there is one, orNone
.
-
get_expiry_date
()¶ Returns the date this session will expire. For sessions with no custom expiration (or those set to expire at browser close), this will equal the date
SESSION_COOKIE_AGE
seconds from now.This function accepts the same keyword arguments as
get_expiry_age()
.
-
get_expire_at_browser_close
()¶ Returns either
True
orFalse
, depending on whether the user’s session cookie will expire when the user’s Web browser is closed.
-
clear_expired
()¶ Removes expired sessions from the session store. This class method is called by
clearsessions
.
-
cycle_key
()¶ Creates a new session key while retaining the current session data.
django.contrib.auth.login()
calls this method to mitigate against session fixation.
-
Session serialization¶
By default, Django serializes session data using JSON. You can use the
SESSION_SERIALIZER
setting to customize the session serialization
format. Even with the caveats described in Write your own serializer, we highly
recommend sticking with JSON serialization especially if you are using the
cookie backend.
For example, here’s an attack scenario if you use pickle
to serialize
session data. If you’re using the signed cookie session backend and SECRET_KEY
is known by an attacker
(there isn’t an inherent vulnerability in Django that would cause it to leak),
the attacker could insert a string into their session which, when unpickled,
executes arbitrary code on the server. The technique for doing so is simple and
easily available on the internet. Although the cookie session storage signs the
cookie-stored data to prevent tampering, a SECRET_KEY
leak
immediately escalates to a remote code execution vulnerability.
Bundled serializers¶
-
class
serializers.
JSONSerializer
¶ A wrapper around the JSON serializer from
django.core.signing
. Can only serialize basic data types.In addition, as JSON supports only string keys, note that using non-string keys in
request.session
won’t work as expected:>>> # initial assignment >>> request.session[0] = 'bar' >>> # subsequent requests following serialization & deserialization >>> # of session data >>> request.session[0] # KeyError >>> request.session['0'] 'bar'
Similarly, data that can’t be encoded in JSON, such as non-UTF8 bytes like
'\xd9'
(which raisesUnicodeDecodeError
), can’t be stored.See the Write your own serializer section for more details on limitations of JSON serialization.
-
class
serializers.
PickleSerializer
¶ Supports arbitrary Python objects, but, as described above, can lead to a remote code execution vulnerability if
SECRET_KEY
becomes known by an attacker.
Write your own serializer¶
Note that unlike PickleSerializer
,
the JSONSerializer
cannot handle
arbitrary Python data types. As is often the case, there is a trade-off between
convenience and security. If you wish to store more advanced data types
including datetime
and Decimal
in JSON backed sessions, you will need
to write a custom serializer (or convert such values to a JSON serializable
object before storing them in request.session
). While serializing these
values is fairly straightforward
(DjangoJSONEncoder
may be helpful),
writing a decoder that can reliably get back the same thing that you put in is
more fragile. For example, you run the risk of returning a datetime
that
was actually a string that just happened to be in the same format chosen for
datetime
s).
Your serializer class must implement two methods,
dumps(self, obj)
and loads(self, data)
, to serialize and deserialize
the dictionary of session data, respectively.
Session object guidelines¶
- Use normal Python strings as dictionary keys on
request.session
. This is more of a convention than a hard-and-fast rule. - Session dictionary keys that begin with an underscore are reserved for internal use by Django.
- Don’t override
request.session
with a new object, and don’t access or set its attributes. Use it like a Python dictionary.
Examples¶
This simplistic view sets a has_commented
variable to True
after a user
posts a comment. It doesn’t let a user post a comment more than once:
def post_comment(request, new_comment):
if request.session.get('has_commented', False):
return HttpResponse("You've already commented.")
c = comments.Comment(comment=new_comment)
c.save()
request.session['has_commented'] = True
return HttpResponse('Thanks for your comment!')
This simplistic view logs in a « member » of the site:
def login(request):
m = Member.objects.get(username=request.POST['username'])
if m.password == request.POST['password']:
request.session['member_id'] = m.id
return HttpResponse("You're logged in.")
else:
return HttpResponse("Your username and password didn't match.")
…And this one logs a member out, according to login()
above:
def logout(request):
try:
del request.session['member_id']
except KeyError:
pass
return HttpResponse("You're logged out.")
The standard django.contrib.auth.logout()
function actually does a bit
more than this to prevent inadvertent data leakage. It calls the
flush()
method of request.session
.
We are using this example as a demonstration of how to work with session
objects, not as a full logout()
implementation.
Setting test cookies¶
As a convenience, Django provides an easy way to test whether the user’s
browser accepts cookies. Just call the
set_test_cookie()
method of
request.session
in a view, and call
test_cookie_worked()
in a subsequent view –
not in the same view call.
This awkward split between set_test_cookie()
and test_cookie_worked()
is necessary due to the way cookies work. When you set a cookie, you can’t
actually tell whether a browser accepted it until the browser’s next request.
It’s good practice to use
delete_test_cookie()
to clean up after
yourself. Do this after you’ve verified that the test cookie worked.
Here’s a typical usage example:
from django.http import HttpResponse
from django.shortcuts import render
def login(request):
if request.method == 'POST':
if request.session.test_cookie_worked():
request.session.delete_test_cookie()
return HttpResponse("You're logged in.")
else:
return HttpResponse("Please enable cookies and try again.")
request.session.set_test_cookie()
return render(request, 'foo/login_form.html')
Using sessions out of views¶
Note
The examples in this section import the SessionStore
object directly
from the django.contrib.sessions.backends.db
backend. In your own code,
you should consider importing SessionStore
from the session engine
designated by SESSION_ENGINE
, as below:
>>> from importlib import import_module
>>> from django.conf import settings
>>> SessionStore = import_module(settings.SESSION_ENGINE).SessionStore
An API is available to manipulate session data outside of a view:
>>> from django.contrib.sessions.backends.db import SessionStore
>>> s = SessionStore()
>>> # stored as seconds since epoch since datetimes are not serializable in JSON.
>>> s['last_login'] = 1376587691
>>> s.create()
>>> s.session_key
'2b1189a188b44ad18c35e113ac6ceead'
>>> s = SessionStore(session_key='2b1189a188b44ad18c35e113ac6ceead')
>>> s['last_login']
1376587691
SessionStore.create()
is designed to create a new session (i.e. one not
loaded from the session store and with session_key=None
). save()
is
designed to save an existing session (i.e. one loaded from the session store).
Calling save()
on a new session may also work but has a small chance of
generating a session_key
that collides with an existing one. create()
calls save()
and loops until an unused session_key
is generated.
If you’re using the django.contrib.sessions.backends.db
backend, each
session is just a normal Django model. The Session
model is defined in
django/contrib/sessions/models.py
. Because it’s a normal model, you can
access sessions using the normal Django database API:
>>> from django.contrib.sessions.models import Session
>>> s = Session.objects.get(pk='2b1189a188b44ad18c35e113ac6ceead')
>>> s.expire_date
datetime.datetime(2005, 8, 20, 13, 35, 12)
Note that you’ll need to call
get_decoded()
to get the session
dictionary. This is necessary because the dictionary is stored in an encoded
format:
>>> s.session_data
'KGRwMQpTJ19hdXRoX3VzZXJfaWQnCnAyCkkxCnMuMTExY2ZjODI2Yj...'
>>> s.get_decoded()
{'user_id': 42}
When sessions are saved¶
By default, Django only saves to the session database when the session has been modified – that is if any of its dictionary values have been assigned or deleted:
# Session is modified.
request.session['foo'] = 'bar'
# Session is modified.
del request.session['foo']
# Session is modified.
request.session['foo'] = {}
# Gotcha: Session is NOT modified, because this alters
# request.session['foo'] instead of request.session.
request.session['foo']['bar'] = 'baz'
In the last case of the above example, we can tell the session object
explicitly that it has been modified by setting the modified
attribute on
the session object:
request.session.modified = True
To change this default behavior, set the SESSION_SAVE_EVERY_REQUEST
setting to True
. When set to True
, Django will save the session to the
database on every single request.
Note that the session cookie is only sent when a session has been created or
modified. If SESSION_SAVE_EVERY_REQUEST
is True
, the session
cookie will be sent on every request.
Similarly, the expires
part of a session cookie is updated each time the
session cookie is sent.
The session is not saved if the response’s status code is 500.
Browser-length sessions vs. persistent sessions¶
You can control whether the session framework uses browser-length sessions vs.
persistent sessions with the SESSION_EXPIRE_AT_BROWSER_CLOSE
setting.
By default, SESSION_EXPIRE_AT_BROWSER_CLOSE
is set to False
,
which means session cookies will be stored in users” browsers for as long as
SESSION_COOKIE_AGE
. Use this if you don’t want people to have to
log in every time they open a browser.
If SESSION_EXPIRE_AT_BROWSER_CLOSE
is set to True
, Django will
use browser-length cookies – cookies that expire as soon as the user closes
their browser. Use this if you want people to have to log in every time they
open a browser.
This setting is a global default and can be overwritten at a per-session level
by explicitly calling the set_expiry()
method
of request.session
as described above in using sessions in views.
Note
Some browsers (Chrome, for example) provide settings that allow users to
continue browsing sessions after closing and re-opening the browser. In
some cases, this can interfere with the
SESSION_EXPIRE_AT_BROWSER_CLOSE
setting and prevent sessions
from expiring on browser close. Please be aware of this while testing
Django applications which have the
SESSION_EXPIRE_AT_BROWSER_CLOSE
setting enabled.
Clearing the session store¶
As users create new sessions on your website, session data can accumulate in
your session store. If you’re using the database backend, the
django_session
database table will grow. If you’re using the file backend,
your temporary directory will contain an increasing number of files.
To understand this problem, consider what happens with the database backend.
When a user logs in, Django adds a row to the django_session
database
table. Django updates this row each time the session data changes. If the user
logs out manually, Django deletes the row. But if the user does not log out,
the row never gets deleted. A similar process happens with the file backend.
Django does not provide automatic purging of expired sessions. Therefore,
it’s your job to purge expired sessions on a regular basis. Django provides a
clean-up management command for this purpose: clearsessions
. It’s
recommended to call this command on a regular basis, for example as a daily
cron job.
Note that the cache backend isn’t vulnerable to this problem, because caches automatically delete stale data. Neither is the cookie backend, because the session data is stored by the users” browsers.
Settings¶
A few Django settings give you control over session behavior:
Session security¶
Subdomains within a site are able to set cookies on the client for the whole domain. This makes session fixation possible if cookies are permitted from subdomains not controlled by trusted users.
For example, an attacker could log into good.example.com
and get a valid
session for their account. If the attacker has control over bad.example.com
,
they can use it to send their session key to you since a subdomain is permitted
to set cookies on *.example.com
. When you visit good.example.com
,
you’ll be logged in as the attacker and might inadvertently enter your
sensitive personal data (e.g. credit card info) into the attacker’s account.
Another possible attack would be if good.example.com
sets its
SESSION_COOKIE_DOMAIN
to "example.com"
which would cause
session cookies from that site to be sent to bad.example.com
.
Technical details¶
- The session dictionary accepts any
json
serializable value when usingJSONSerializer
or any picklable Python object when usingPickleSerializer
. See thepickle
module for more information. - Session data is stored in a database table named
django_session
. - Django only sends a cookie if it needs to. If you don’t set any session data, it won’t send a session cookie.
The SessionStore
object¶
When working with sessions internally, Django uses a session store object from
the corresponding session engine. By convention, the session store object class
is named SessionStore
and is located in the module designated by
SESSION_ENGINE
.
All SessionStore
classes available in Django inherit from
SessionBase
and implement data manipulation methods,
namely:
exists()
create()
save()
delete()
load()
clear_expired()
In order to build a custom session engine or to customize an existing one, you
may create a new class inheriting from SessionBase
or
any other existing SessionStore
class.
Extending most of the session engines is quite straightforward, but doing so with database-backed session engines generally requires some extra effort (see the next section for details).
Extending database-backed session engines¶
Creating a custom database-backed session engine built upon those included in
Django (namely db
and cached_db
) may be done by inheriting
AbstractBaseSession
and either SessionStore
class.
AbstractBaseSession
and BaseSessionManager
are importable from
django.contrib.sessions.base_session
so that they can be imported without
including django.contrib.sessions
in INSTALLED_APPS
.
-
class
base_session.
AbstractBaseSession
¶ The abstract base session model.
-
session_key
¶ Primary key. The field itself may contain up to 40 characters. The current implementation generates a 32-character string (a random sequence of digits and lowercase ASCII letters).
-
session_data
¶ A string containing an encoded and serialized session dictionary.
-
expire_date
¶ A datetime designating when the session expires.
Expired sessions are not available to a user, however, they may still be stored in the database until the
clearsessions
management command is run.
-
classmethod
get_session_store_class
()¶ Returns a session store class to be used with this session model.
-
get_decoded
()¶ Returns decoded session data.
Decoding is performed by the session store class.
-
You can also customize the model manager by subclassing
BaseSessionManager
:
-
class
base_session.
BaseSessionManager
¶ -
encode
(session_dict)¶ Returns the given session dictionary serialized and encoded as a string.
Encoding is performed by the session store class tied to a model class.
-
save
(session_key, session_dict, expire_date)¶ Saves session data for a provided session key, or deletes the session in case the data is empty.
-
Customization of SessionStore
classes is achieved by overriding methods
and properties described below:
-
class
backends.db.
SessionStore
¶ Implements database-backed session store.
-
classmethod
get_model_class
()¶ Override this method to return a custom session model if you need one.
-
create_model_instance
(data)¶ Returns a new instance of the session model object, which represents the current session state.
Overriding this method provides the ability to modify session model data before it’s saved to database.
-
classmethod
-
class
backends.cached_db.
SessionStore
¶ Implements cached database-backed session store.
-
cache_key_prefix
¶ A prefix added to a session key to build a cache key string.
-
Example¶
The example below shows a custom database-backed session engine that includes an additional database column to store an account ID (thus providing an option to query the database for all active sessions for an account):
from django.contrib.sessions.backends.db import SessionStore as DBStore
from django.contrib.sessions.base_session import AbstractBaseSession
from django.db import models
class CustomSession(AbstractBaseSession):
account_id = models.IntegerField(null=True, db_index=True)
@classmethod
def get_session_store_class(cls):
return SessionStore
class SessionStore(DBStore):
@classmethod
def get_model_class(cls):
return CustomSession
def create_model_instance(self, data):
obj = super().create_model_instance(data)
try:
account_id = int(data.get('_auth_user_id'))
except (ValueError, TypeError):
account_id = None
obj.account_id = account_id
return obj
If you are migrating from the Django’s built-in cached_db
session store to
a custom one based on cached_db
, you should override the cache key prefix
in order to prevent a namespace clash:
class SessionStore(CachedDBStore):
cache_key_prefix = 'mysessions.custom_cached_db_backend'
# ...
Session IDs in URLs¶
The Django sessions framework is entirely, and solely, cookie-based. It does not fall back to putting session IDs in URLs as a last resort, as PHP does. This is an intentional design decision. Not only does that behavior make URLs ugly, it makes your site vulnerable to session-ID theft via the « Referer » header.