Cross Site Request Forgery (CSRF)
Spring provides comprehensive support for protecting against Cross Site Request Forgery (CSRF) attacks. In the following sections, we explore:
What is a CSRF Attack?
The best way to understand a CSRF attack is by taking a look at a concrete example.
Assume that your bank’s website provides a form that allows transferring money from the currently logged in user to another bank account. For example, the transfer form might look like:
<form method="post"
action="/transfer">
<input type="text"
name="amount"/>
<input type="text"
name="routingNumber"/>
<input type="text"
name="account"/>
<input type="submit"
value="Transfer"/>
</form>The corresponding HTTP request might look like:
POST /transfer HTTP/1.1
Host: bank.example.com
Cookie: JSESSIONID=randomid
Content-Type: application/x-www-form-urlencoded
amount=100.00&routingNumber=1234&account=9876Now pretend you authenticate to your bank’s website and then, without logging out, visit an evil website. The evil website contains an HTML page with the following form:
<form method="post"
action="https://bank.example.com/transfer">
<input type="hidden"
name="amount"
value="100.00"/>
<input type="hidden"
name="routingNumber"
value="evilsRoutingNumber"/>
<input type="hidden"
name="account"
value="evilsAccountNumber"/>
<input type="submit"
value="Win Money!"/>
</form>You like to win money, so you click on the submit button. In the process, you have unintentionally transferred $100 to a malicious user. This happens because, while the evil website cannot see your cookies, the cookies associated with your bank are still sent along with the request.
Worse yet, this whole process could have been automated by using JavaScript. This means you did not even need to click on the button. Furthermore, it could just as easily happen when visiting an honest site that is a victim of a XSS attack. So how do we protect our users from such attacks?
Protecting Against CSRF Attacks
The reason that a CSRF attack is possible is that the HTTP request from the victim’s website and the request from the attacker’s website are exactly the same. This means there is no way to reject requests coming from the evil website and allow only requests coming from the bank’s website. To protect against CSRF attacks, we need to ensure there is something in the request that the evil site is unable to provide so we can differentiate the two requests.
Spring provides two mechanisms to protect against CSRF attacks:
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Specifying the SameSite Attribute on your session cookie
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Both protections require that Safe Methods be Read-only. |
Safe Methods Must be Read-only
For either protection against CSRF to work, the application must ensure that "safe" HTTP methods are read-only.
This means that requests with the HTTP GET, HEAD, OPTIONS, and TRACE methods should not change the state of the application.
Synchronizer Token Pattern
The predominant and most comprehensive way to protect against CSRF attacks is to use the Synchronizer Token Pattern. This solution is to ensure that each HTTP request requires, in addition to our session cookie, a secure random generated value called a CSRF token be present in the HTTP request.
When an HTTP request is submitted, the server must look up the expected CSRF token and compare it against the actual CSRF token in the HTTP request. If the values do not match, the HTTP request should be rejected.
The key to this working is that the actual CSRF token should be in a part of the HTTP request that is not automatically included by the browser. For example, requiring the actual CSRF token in an HTTP parameter or an HTTP header will protect against CSRF attacks. Requiring the actual CSRF token in a cookie does not work because cookies are automatically included in the HTTP request by the browser.
We can relax the expectations to require only the actual CSRF token for each HTTP request that updates the state of the application. For that to work, our application must ensure that safe HTTP methods are read-only. This improves usability, since we want to allow linking to our website from external sites. Additionally, we do not want to include the random token in HTTP GET, as this can cause the tokens to be leaked.
Consider how our example would change when we use the Synchronizer Token Pattern.
Assume that the actual CSRF token is required to be in an HTTP parameter named _csrf.
Our application’s transfer form would look like:
<form method="post"
action="/transfer">
<input type="hidden"
name="_csrf"
value="4bfd1575-3ad1-4d21-96c7-4ef2d9f86721"/>
<input type="text"
name="amount"/>
<input type="text"
name="routingNumber"/>
<input type="hidden"
name="account"/>
<input type="submit"
value="Transfer"/>
</form>The form now contains a hidden input with the value of the CSRF token. External sites cannot read the CSRF token since the same origin policy ensures the evil site cannot read the response.
The corresponding HTTP request to transfer money would look like this:
POST /transfer HTTP/1.1
Host: bank.example.com
Cookie: JSESSIONID=randomid
Content-Type: application/x-www-form-urlencoded
amount=100.00&routingNumber=1234&account=9876&_csrf=4bfd1575-3ad1-4d21-96c7-4ef2d9f86721You will notice that the HTTP request now contains the _csrf parameter with a secure random value.
The evil website will not be able to provide the correct value for the _csrf parameter (which must be explicitly provided on the evil website) and the transfer will fail when the server compares the actual CSRF token to the expected CSRF token.
SameSite Attribute
An emerging way to protect against CSRF Attacks is to specify the SameSite Attribute on cookies.
A server can specify the SameSite attribute when setting a cookie to indicate that the cookie should not be sent when coming from external sites.
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Spring Security does not directly control the creation of the session cookie, so it does not provide support for the SameSite attribute.
Spring Session provides support for the |
An example, of an HTTP response header with the SameSite attribute might look like:
Set-Cookie: JSESSIONID=randomid; Domain=bank.example.com; Secure; HttpOnly; SameSite=LaxValid values for the SameSite attribute are:
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Strict: When specified, any request coming from the same-site includes the cookie. Otherwise, the cookie is not included in the HTTP request. -
Lax: When specified, cookies are sent when coming from the same-site or when the request comes from top-level navigations and the method is read-only. Otherwise, the cookie is not included in the HTTP request.
Consider how our example could be protected using the SameSite attribute.
The bank application can protect against CSRF by specifying the SameSite attribute on the session cookie.
With the SameSite attribute set on our session cookie, the browser continues to send the JSESSIONID cookie with requests coming from the banking website.
However, the browser no longer sends the JSESSIONID cookie with a transfer request coming from the evil website.
Since the session is no longer present in the transfer request coming from the evil website, the application is protected from the CSRF attack.
There are some important considerations to be aware of when using SameSite attribute to protect against CSRF attacks.
Setting the SameSite attribute to Strict provides a stronger defense but can confuse users.
Consider a user who stays logged into a social media site hosted at social.example.com.
The user receives an email at email.example.org that includes a link to the social media site.
If the user clicks on the link, they would rightfully expect to be authenticated to the social media site.
However, if the SameSite attribute is Strict, the cookie would not be sent and so the user would not be authenticated.
Another obvious consideration is that, in order for the SameSite attribute to protect users, the browser must support the SameSite attribute.
Most modern browsers do support the SameSite attribute.
However, older browsers that are still in use may not.
For this reason, we generally recommend using the SameSite attribute as a defense in depth rather than the sole protection against CSRF attacks.
When to use CSRF protection
When should you use CSRF protection? Our recommendation is to use CSRF protection for any request that could be processed by a browser by normal users. If you are creating a service that is used only by non-browser clients, you likely want to disable CSRF protection.
CSRF protection and JSON
A common question is “do I need to protect JSON requests made by JavaScript?” The short answer is: It depends. However, you must be very careful, as there are CSRF exploits that can impact JSON requests. For example, a malicious user can create a CSRF with JSON by using the following form:
<form action="https://bank.example.com/transfer" method="post" enctype="text/plain">
<input name='{"amount":100,"routingNumber":"evilsRoutingNumber","account":"evilsAccountNumber", "ignore_me":"' value='test"}' type='hidden'>
<input type="submit"
value="Win Money!"/>
</form>This produces the following JSON structure
{ "amount": 100,
"routingNumber": "evilsRoutingNumber",
"account": "evilsAccountNumber",
"ignore_me": "=test"
}If an application were not validating the Content-Type header, it would be exposed to this exploit.
Depending on the setup, a Spring MVC application that validates the Content-Type could still be exploited by updating the URL suffix to end with .json, as follows:
<form action="https://bank.example.com/transfer.json" method="post" enctype="text/plain">
<input name='{"amount":100,"routingNumber":"evilsRoutingNumber","account":"evilsAccountNumber", "ignore_me":"' value='test"}' type='hidden'>
<input type="submit"
value="Win Money!"/>
</form>CSRF and Stateless Browser Applications
What if my application is stateless? That does not necessarily mean you are protected. In fact, if a user does not need to perform any actions in the web browser for a given request, they are likely still vulnerable to CSRF attacks.
For example, consider an application that uses a custom cookie that contains all the state within it for authentication (instead of the JSESSIONID). When the CSRF attack is made, the custom cookie is sent with the request in the same manner that the JSESSIONID cookie was sent in our previous example. This application is vulnerable to CSRF attacks.
Applications that use basic authentication are also vulnerable to CSRF attacks. The application is vulnerable since the browser automatically includes the username and password in any requests in the same manner that the JSESSIONID cookie was sent in our previous example.