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This lack middleware provides some level of security for your webapp, preventing a CSRF attacks. It has a function
csrf-html-tag which returns a hidden
input element to embed into a form.
The input stores a special token. Middleware saves this token into the current session and ensures the user sends this token in the following requests. If he doesn't, a 400 status code will be returned.
Let's take our yesterday's app and make it more secure!
First, we need to rewrite our main application to make it render a login form with CSRF token. Pay attention to how does it call a
csrf-html-tag function at the end.
If you are going to develop an application with a lots of forms, then it is good idea to define a macro which will apply CSRF protection automatically.
POFTHEDAY> (defun main (env) (let* ((session (getf env :lack.session)) (login (gethash :login session))) (cond (login (list 200 (list :content-type "text/plain") (list (format nil "Welcome, ~A!" login)))) (t (list 200 (list :content-type "text/plain") (list (format nil " <form method=\"POST\" action=\"/login\"> <input type=\"text\" name=\"login\"></input> <input type=\"password\" name=\"password\"></input> ~A </form> " (lack.middleware.csrf:csrf-html-tag session))))))))
All other apps remain the same, we only need to build the whole app including the
This middleware should go after the
:session middleware, because it depends on it:
POFTHEDAY> (clack:clackup (lack:builder :session :csrf (:mount "/login" 'login) (:mount "/logout" 'logout) 'main) :port 8091) Hunchentoot server is started. Listening on 127.0.0.1:8091.
This is how our form is rendered. Note a "hidden" input at the end of the form:
POFTHEDAY> (dex:get "http://localhost:8091/") " <form method=\"POST\" action=\"/login\"> <input type=\"text\" name=\"login\"></input> <input type=\"password\" name=\"password\"></input> <input type=\"hidden\" name=\"_csrf_token\" value=\"8de1c8a47\"> </form>
If we try to do a
POST request without the token, we'll receive a 400 error:
POFTHEDAY> (handler-case (dex:post "http://localhost:8091/login" :content '(("login" . "bob") ("password" . "$ecret")) :headers '((:cookie . "lack.session=75bccc"))) (dexador:http-request-failed (c) (values (dexador:response-status c) (dexador:response-body c)))) 400 "Bad Request: invalid CSRF token"
Using the code we'll be able to log in:
POFTHEDAY> (dex:post "http://localhost:8091/login" :content '(("login" . "bob") ("password" . "$ecret") ("_csrf_token" . "8de1c8a47")) :headers '((:cookie . "lack.session=75bccc"))) "Dear Bob, you welcome!" 200
The middleware also has a few settings.
You can set
:session-key to a value other than
_csrf_token. But this changes only a token's key inside the session. Form field's name remains the
Other option is
:one-time. Set it to true if you want to remove a token from the session after the first successful
And finally, you can define your own handler for the error page and pass it as ":block-app". It should be a usual Clack app.