Dan Poirier wrote an article on the Caktus Group blog about common web site security vulnerabilities. In it he talked about the CSRF protection in Django. Although he is right about a CSRF token having to be part of the POST request, this is not the entire story.
It is not my intention to claim that mister Poirier does not know how the CSRF protection in Django works. I only want to present a more complete version.
First things first, for those of you that have not read the Dan Poiriers article, here’s a short summary of the CSRF related parts.
Cross-site request forgery (CSRF or XSRF) is a type of attack where a malicious site is trying to make your browser send requests to another site in an attempt to leverage the permissions of the user—you. (For more information and examples, check the original article or the OWASP page on CSRF.)
Besides making sure that GET requests do not change data the article talks about the CSRF protection provided by Django. Specifically it states the following (emphasis mine):
Django’s protection is to always include a user-specific, unguessable string as part of such requests, and reject any such request that doesn’t include it. This string is called the CSRF token. Any form on a Django site that does a POST etc has to include it as one of the submitted parameters. Since the malicious site doesn’t know the token, it cannot generate a malicious POST request that the Django site will pay any attention to.
This is where the author is not incorrect (the POST request indeed has to include the CSRF token), but this is only one half of the mechanism. The other half is a cookie that is set by the original site where the user is logged in. Only when the server receives both values from the browser (and they match) will the POST request be valid. This is described as the “Double Submit Cookie” defence in the Cross-Site Request Forgery (CSRF) Prevention Cheat Sheet by OWASP.
Here’s an example with a very simple form (source code on GitHub). Let’s first request the form:
$ curl -i http://localhost:8000/post_to_me/
The response will look something like this:
HTTP/1.0 200 OK Date: Tue, 15 Nov 2016 20:25:58 GMT Server: WSGIServer/0.2 CPython/3.4.3 Content-Type: text/html; charset=utf-8 Vary: Cookie X-Frame-Options: SAMEORIGIN Set-Cookie: csrftoken=wVFdNQ1Hz487w7yk2mVjre2qlsclXi99w2jEcGyvXorojDLd7jH09NGhmbavG3tx; expires=Tue, 14-Nov-2017 20:55:58 GMT; Max-Age=31449600; Path=/ <!doctype html> <html> <body> <form action="/post_to_me/" method="post"> <input type='hidden' name='csrfmiddlewaretoken' value='JBWuPGvKU54xW9YIwEIknst1azSkBmg3JIAVew2yipnOJFbBBBu1517SbiQuk7Ar' /> <tr><th><label for="id_name">Name:</label></th><td><input id="id_name" name="name" type="text" required /></td></tr> <input type="submit" value="Post" /> </form> </body> </html>
csrftoken value in the
Set-Cookie header. Also note the
csrfmiddlewaretoken value in the form in the body of the
response. We’ll use these values in our examples where we send POST requests.
First a demonstration that we can successfully post a value using both
the cookie and the value in the form (in the
$ curl -s -D - -o /dev/null \ -H 'Cookie: csrftoken=wVFdNQ1Hz487w7yk2mVjre2qlsclXi99w2jEcGyvXorojDLd7jH09NGhmbavG3tx' \ --data 'csrfmiddlewaretoken=JBWuPGvKU54xW9YIwEIknst1azSkBmg3JIAVew2yipnOJFbBBBu1517SbiQuk7Ar&name=mark' \ http://localhost:8000/post_to_me/
The response is a nice
HTTP/1.0 200 OK Date: Tue, 15 Nov 2016 20:30:35 GMT Server: WSGIServer/0.2 CPython/3.4.3 X-Frame-Options: SAMEORIGIN Content-Type: text/html; charset=utf-8
If we do not send the cookie along, we expect that the POST request will fail:
$ curl -s -D - -o /dev/null \ --data 'csrfmiddlewaretoken=JBWuPGvKU54xW9YIwEIknst1azSkBmg3JIAVew2yipnOJFbBBBu1517SbiQuk7Ar&name=mark' \ http://localhost:8000/post_to_me/
And indeed the server responds with a
403 Forbidden status:
HTTP/1.0 403 Forbidden Date: Tue, 15 Nov 2016 20:32:47 GMT Server: WSGIServer/0.2 CPython/3.4.3 X-Frame-Options: SAMEORIGIN Content-Type: text/html
We could try to include the cookie but leave out the value in the form to check if the cookie alone is enough:
curl -s -D - -o /dev/null \ -H 'Cookie: csrftoken=wVFdNQ1Hz487w7yk2mVjre2qlsclXi99w2jEcGyvXorojDLd7jH09NGhmbavG3tx' \ --data 'name=mark' \ http://localhost:8000/post_to_me/
However, this has the same result:
HTTP/1.0 403 Forbidden Date: Tue, 15 Nov 2016 20:33:35 GMT Server: WSGIServer/0.2 CPython/3.4.3 X-Frame-Options: SAMEORIGIN Content-Type: text/html
As you can see Django requires both components to be present. The actual value of the token is less relevant. Sure, it is “unguessable”, but that is (in my humble opinion) not the most relevant part. The CSRF token is also not stored. Django could not care that much about the actual value—as long as the value in the cookie matches the one in the POST data, the token is considered valid.
To demonstrate that I can make up my own values if I want to (as long as they are 32 or 64 characters in length):
$ curl -s -D - -o /dev/null \ -H 'Cookie: csrftoken= markmarkmarkmarkmarkmarkmarkmark' \ --data 'csrfmiddlewaretoken=markmarkmarkmarkmarkmarkmarkmark&name=mark' \ http://localhost:8000/post_to_me/
HTTP/1.0 200 OK Date: Tue, 15 Nov 2016 20:35:55 GMT Server: WSGIServer/0.2 CPython/3.4.3 X-Frame-Options: SAMEORIGIN Content-Type: text/html; charset=utf-8
This example uses an older version of the CSRF token. As of Django 1.10 the CSRF form token value is salted and changed every request. (For details, see this commit and the related issue.) This does not change the mechanism behind the defence though.
Why does this defence work?
Back to the attack. Why does it matter that the POST request has to have matching tokens both via the cookie and the POST data?
Cookies are set for a specific domain. Your browser protects this in two ways:
- You cannot set a cookie for a different domain.
- Cookies for one domain are not sent to another domain.
This defence against CSRF works because although the evil site can force the browser to make a request to the site it wants to abuse, the attacker can only manipulate the request and its data. The attacker cannot set, modify or even read a cookie for a different domain than its own. As a result the attacker cannot determine or even guess which CSRF token should be in the request and thus the request will fail.
Note that this is the protection chosen by Django. Other forms of defence are possible. See the aforementioned Cross-Site Request Forgery (CSRF) Prevention Cheat Sheet for more information.
As already stated at the top of the article, I do not want to imply that mister Poirier does not know how the CSRF protection in Django works; perhaps he decided to leave out some of these details to make his article more succinct. Either way, in my opinion his article only told half of the story with regards to CSRF protection. So I decided to talk about the other half.