What Are Anti Cross-site Request Forgery Tokens And What Are They Good For?
Anti Cross-site Request Forgery Tokens help prevent Cross-site Request Forgery (CSRF) also known as XSRF – pronounced “sea-surf” – and are usually implemented through a hidden HTML form element that contains a unique ID. This ID is passed along with subsequent requests for data and validated on the server. Anti CSRF Tokens try to ensure the identity of the user. They aren’t a replacement for CAPTCHAs and don’t prevent robots or web scrapers from manipulating your site – as you’ll soon see.
Why use an Anti CRSF Token?
An overly simple example: If I didn’t use an Anti Forgery Token on my contact page (see the source code: View or Controller), a Spammer could POST data directly against my contact form and potentially drown me with spam.
Here’s a hypothetical form created by an evil Spammer. This form is hosted on http://spammer.com (not my site):
<form action="http://adam.kahtava.com/contact/send" method="POST"> <input name="fromName" type="text" value="Johnathon Fink" /> <input name="fromAddress" type="text" value="firstname.lastname@example.org" /> <input name="subject" type="text" value="Call for your diploma now" /> <textarea name="body">Is your lack of a degree...</textarea> ... </form>
Again, note that the form action contains a reference to my site (even though it is hosted on another site).
Now, imagine this was a form prompting a user for their username and password. These credentials could be maliciously stored while the user successfully authenticates and is then redirected to the site they thought they were visiting – the way phishing usually works.
After adding an Anti CRSF Token to my contact form, a Spammer can’t access my form remotely (at least not without the token). My contact form with it’s Anti CRSF Token:
<form action="/contact/send" method="post" name="contact"> <input name="__RequestVerificationToken" type="hidden" value="0sAqY1ZKb+Qia4..." /> <input name="fromName" ...
Note the presence of the RequestVerificationToken.
Said Spammer, can’t abuse my form without including the unique token. Technically speaking the Spammer can still abuse my form, but he now needs to:
- Request (GET) my contact form
- Parse out the Anti CRSF Token
- POST their spam into my contact form along with the token
This is pretty easy to do if you have an implementation of a HTTP Client library that supports cookies.
How to hack an Anti CRSF Token protected form
Using an extended instance of .NETs Web Client here’s how our Spammer could circumvent my Anti CRSF Token.
The Spamming script by that wascaly Spammer:
// create a new HTTP Web Client that supports cookies var webClient = new WebClientWithCookies(); //download my contact page containing the Anti CRSF Token webClient = webClient.DownloadData("http://adam.kahtava.com/contact"); //parse out the Anti CRSF Token var antiCrsfToken = RegexUtilities.GetTokenString( new Regex("__RequestVerificationToken=(?<CRSF_Token>[^;]+)") .Match(webClient.ResponseHeaders["Set-Cookie"]), "CRSF_Token"); //now the Spammer can drown me in spam-spam-spam // by scraping my Anti CRSF Token and posting it into my form webClient.Headers.Add("Content-Type", "application/x-www-form-urlencoded"); byte response = webClient.UploadData("http://adam.kahtava.com/contact/send", "POST", Encoding.UTF8.GetBytes( "__RequestVerificationToken=" + antiCrsfToken + "&fromName=\"Johnathon Fink\"" + "&fromAddress=\"email@example.com\"" + "&subject=\"Call for your diploma now\"" + "&body=\"Is your lack of a degree...\""));
The Spammer is back at their old tricks sending me more Spam. ARGH!
What’s the use of an Anti CRSF Token?
Anti CRSF Tokens help prevent phishing attacks. They aren’t meant to prevent spammers or Dr Robotnik and his robots (or web scrapers) from running automated scripts against your web application. Keep in mind, that if your site suffers from other XSS vulnerabilities (where the privacy of your cookies or sessions are compromised) then Anti CRSF Tokens don’t work at all.
Read more about how Anti CRSF Tokens work here: Prevent Cross-Site Request Forgery (CSRF) using ASP.NET MVC’s AntiForgeryToken() helper or learn more about Cross-Site Request Forgery at: The Cross-Site Request Forgery (CSRF/XSRF) FAQ.