Authentication and security¶
Cookies and secure cookies¶
You can set cookies in the user’s browser with the set_cookie
method:
class MainHandler(tornado.web.RequestHandler):
def get(self):
if not self.get_cookie("mycookie"):
self.set_cookie("mycookie", "myvalue")
self.write("Your cookie was not set yet!")
else:
self.write("Your cookie was set!")
Cookies are not secure and can easily be modified by clients. If you
need to set cookies to, e.g., identify the currently logged in user,
you need to sign your cookies to prevent forgery. Tornado supports
signed cookies with the set_secure_cookie and
get_secure_cookie methods. To use these methods,
you need to specify a secret key named cookie_secret when you
create your application. You can pass in application settings as
keyword arguments to your application:
application = tornado.web.Application([
(r"/", MainHandler),
], cookie_secret="__TODO:_GENERATE_YOUR_OWN_RANDOM_VALUE_HERE__")
Signed cookies contain the encoded value of the cookie in addition to a
timestamp and an HMAC signature.
If the cookie is old or if the signature doesn’t match,
get_secure_cookie will return None just as if the cookie isn’t
set. The secure version of the example above:
class MainHandler(tornado.web.RequestHandler):
def get(self):
if not self.get_secure_cookie("mycookie"):
self.set_secure_cookie("mycookie", "myvalue")
self.write("Your cookie was not set yet!")
else:
self.write("Your cookie was set!")
Tornado’s secure cookies guarantee integrity but not confidentiality.
That is, the cookie cannot be modified but its contents can be seen by the
user. The cookie_secret is a symmetric key and must be kept secret –
anyone who obtains the value of this key could produce their own signed
cookies.
By default, Tornado’s secure cookies expire after 30 days. To change this,
use the expires_days keyword argument to set_secure_cookie and the
max_age_days argument to get_secure_cookie. These two values are
passed separately so that you may e.g. have a cookie that is valid for 30 days
for most purposes, but for certain sensitive actions (such as changing billing
information) you use a smaller max_age_days when reading the cookie.
Tornado also supports multiple signing keys to enable signing key
rotation. cookie_secret then must be a dict with integer key versions
as keys and the corresponding secrets as values. The currently used
signing key must then be set as key_version application setting
but all other keys in the dict are allowed for cookie signature validation,
if the correct key version is set in the cookie.
To implement cookie updates, the current signing key version can be
queried via get_secure_cookie_key_version.
User authentication¶
The currently authenticated user is available in every request handler
as self.current_user, and in every
template as current_user. By default, current_user is
None.
To implement user authentication in your application, you need to
override the get_current_user() method in your request handlers to
determine the current user based on, e.g., the value of a cookie. Here
is an example that lets users log into the application simply by
specifying a nickname, which is then saved in a cookie:
class BaseHandler(tornado.web.RequestHandler):
def get_current_user(self):
return self.get_secure_cookie("user")
class MainHandler(BaseHandler):
def get(self):
if not self.current_user:
self.redirect("/login")
return
name = tornado.escape.xhtml_escape(self.current_user)
self.write("Hello, " + name)
class LoginHandler(BaseHandler):
def get(self):
self.write('<html><body><form action="/login" method="post">'
'Name: <input type="text" name="name">'
'<input type="submit" value="Sign in">'
'</form></body></html>')
def post(self):
self.set_secure_cookie("user", self.get_argument("name"))
self.redirect("/")
application = tornado.web.Application([
(r"/", MainHandler),
(r"/login", LoginHandler),
], cookie_secret="__TODO:_GENERATE_YOUR_OWN_RANDOM_VALUE_HERE__")
You can require that the user be logged in using the Python
decorator
tornado.web.authenticated. If a request goes to a method with this
decorator, and the user is not logged in, they will be redirected to
login_url (another application setting). The example above could be
rewritten:
class MainHandler(BaseHandler):
@tornado.web.authenticated
def get(self):
name = tornado.escape.xhtml_escape(self.current_user)
self.write("Hello, " + name)
settings = {
"cookie_secret": "__TODO:_GENERATE_YOUR_OWN_RANDOM_VALUE_HERE__",
"login_url": "/login",
}
application = tornado.web.Application([
(r"/", MainHandler),
(r"/login", LoginHandler),
], **settings)
If you decorate post() methods with the authenticated
decorator, and the user is not logged in, the server will send a
403 response. The @authenticated decorator is simply
shorthand for if not self.current_user: self.redirect() and may
not be appropriate for non-browser-based login schemes.
Check out the Tornado Blog example application for a complete example that uses authentication (and stores user data in a MySQL database).
Third party authentication¶
The tornado.auth module implements the authentication and
authorization protocols for a number of the most popular sites on the
web, including Google/Gmail, Facebook, Twitter, and FriendFeed.
The module includes methods to log users in via these sites and, where
applicable, methods to authorize access to the service so you can, e.g.,
download a user’s address book or publish a Twitter message on their
behalf.
Here is an example handler that uses Google for authentication, saving the Google credentials in a cookie for later access:
class GoogleOAuth2LoginHandler(tornado.web.RequestHandler,
tornado.auth.GoogleOAuth2Mixin):
@tornado.gen.coroutine
def get(self):
if self.get_argument('code', False):
user = yield self.get_authenticated_user(
redirect_uri='http://your.site.com/auth/google',
code=self.get_argument('code'))
# Save the user with e.g. set_secure_cookie
else:
yield self.authorize_redirect(
redirect_uri='http://your.site.com/auth/google',
client_id=self.settings['google_oauth']['key'],
scope=['profile', 'email'],
response_type='code',
extra_params={'approval_prompt': 'auto'})
See the tornado.auth module documentation for more details.
Cross-site request forgery protection¶
Cross-site request forgery, or XSRF, is a common problem for personalized web applications. See the Wikipedia article for more information on how XSRF works.
The generally accepted solution to prevent XSRF is to cookie every user with an unpredictable value and include that value as an additional argument with every form submission on your site. If the cookie and the value in the form submission do not match, then the request is likely forged.
Tornado comes with built-in XSRF protection. To include it in your site,
include the application setting xsrf_cookies:
settings = {
"cookie_secret": "__TODO:_GENERATE_YOUR_OWN_RANDOM_VALUE_HERE__",
"login_url": "/login",
"xsrf_cookies": True,
}
application = tornado.web.Application([
(r"/", MainHandler),
(r"/login", LoginHandler),
], **settings)
If xsrf_cookies is set, the Tornado web application will set the
_xsrf cookie for all users and reject all POST, PUT, and
DELETE requests that do not contain a correct _xsrf value. If
you turn this setting on, you need to instrument all forms that submit
via POST to contain this field. You can do this with the special
UIModule xsrf_form_html(), available in all templates:
<form action="/new_message" method="post">
{% module xsrf_form_html() %}
<input type="text" name="message"/>
<input type="submit" value="Post"/>
</form>
If you submit AJAX POST requests, you will also need to instrument
your JavaScript to include the _xsrf value with each request. This
is the jQuery function we use at FriendFeed for
AJAX POST requests that automatically adds the _xsrf value to
all requests:
function getCookie(name) {
var r = document.cookie.match("\\b" + name + "=([^;]*)\\b");
return r ? r[1] : undefined;
}
jQuery.postJSON = function(url, args, callback) {
args._xsrf = getCookie("_xsrf");
$.ajax({url: url, data: $.param(args), dataType: "text", type: "POST",
success: function(response) {
callback(eval("(" + response + ")"));
}});
};
For PUT and DELETE requests (as well as POST requests that
do not use form-encoded arguments), the XSRF token may also be passed
via an HTTP header named X-XSRFToken. The XSRF cookie is normally
set when xsrf_form_html is used, but in a pure-Javascript application
that does not use any regular forms you may need to access
self.xsrf_token manually (just reading the property is enough to
set the cookie as a side effect).
If you need to customize XSRF behavior on a per-handler basis, you can
override RequestHandler.check_xsrf_cookie(). For example, if you
have an API whose authentication does not use cookies, you may want to
disable XSRF protection by making check_xsrf_cookie() do nothing.
However, if you support both cookie and non-cookie-based authentication,
it is important that XSRF protection be used whenever the current
request is authenticated with a cookie.