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=9876

Now 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.

Worst yet, this whole process could have been automated using JavaScript. This means you didn’t 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?

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 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:

	Note
	Both protections require that the section called “Safe Methods Must be Idempotent”

<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>

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=4bfd1575-3ad1-4d21-96c7-4ef2d9f86721

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.

	Note
	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 SameSite attribute in servlet based applications. Spring Framework’s CookieWebSessionIdResolver provides out of the box support for the SameSite attribute in WebFlux based applications.

An example, HTTP response header with the SameSite attribute might look like:

Example 5.6. SameSite HTTP response

Set-Cookie: JSESSIONID=randomid; Domain=bank.example.com; Secure; HttpOnly; SameSite=Lax

Valid values for the SameSite attribute are:

• Strict - when specified any request coming from the same-site will include the cookie. Otherwise, the cookie will not be included in the HTTP request.
• Lax - when specified cookies will be sent when coming from the same-site or when the request comes from top-level navigations and the method is idempotent. Otherwise, the cookie will not be included in the HTTP request.

Let’s take a look at 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 will continue to send the JSESSIONID cookie with requests coming from the banking website. However, the browser will no longer send 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 that one should be aware about 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 that stays logged into a social media site hosted at https://social.example.com. The user receives an email at https://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.

	Note
	We could improve the protection and usability of SameSite protection against CSRF attacks by implementing gh-7537.

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, it is generally recommended to use the SameSite attribute as a defense in depth rather than the sole protection against CSRF attacks.

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 only creating a service that is used by non-browser clients, you will likely want to disable CSRF protection.

<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>

{ "amount": 100,
"routingNumber": "evilsRoutingNumber",
"account": "evilsAccountNumber",
"ignore_me": "=test"
}

<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>

There are a few special considerations to consider when implementing protection against CSRF attacks.

Multipart (file upload)

Protecting multipart requests (file uploads) from CSRF attacks causes a chicken and the egg problem. In order to prevent a CSRF attack from occurring, the body of the HTTP request must be read to obtain actual CSRF token. However, reading the body means that the file will be uploaded which means an external site can upload a file.

There are two options to using CSRF protection with multipart/form-data. Each option has its trade-offs.

• Place CSRF Token in the Body
• Place CSRF Token in the URL

	Note
	Before you integrate Spring Security’s CSRF protection with multipart file upload, ensure that you can upload without the CSRF protection first. More information about using multipart forms with Spring can be found within the 1.1.11. Multipart Resolver section of the Spring reference and the MultipartFilter javadoc.

Place CSRF Token in the Body

The first option is to include the actual CSRF token in the body of the request. By placing the CSRF token in the body, the body will be read before authorization is performed. This means that anyone can place temporary files on your server. However, only authorized users will be able to submit a File that is processed by your application. In general, this is the recommended approach because the temporary file uplaod should have a negligible impact on most servers.

Include CSRF Token in URL

If allowing unauthorized users to upload temporary files is not acceptable, an alternative is to include the expected CSRF token as a query parameter in the action attribute of the form. The disadvantage to this approach is that query parameters can be leaked. More generally, it is considered best practice to place sensitive data within the body or headers to ensure it is not leaked. Additional information can be found in RFC 2616 Section 15.1.3 Encoding Sensitive Information in URI’s.

HiddenHttpMethodFilter

In some applications a form parameter can be used to override the HTTP method. For example, the form below could be used to treat the HTTP method as a delete rather than a post.

Example 5.10. CSRF Hidden HTTP Method Form

<form action="/process"
    method="post">
    <!-- ... -->
    <input type="hidden"
        name="_method"
        value="delete"/>
</form>

Overriding the HTTP method occurs in a filter. That filter must be placed before Spring Security’s support. Note that overriding only happens on a post, so this is actually unlikely to cause any real problems. However, it is still best practice to ensure it is placed before Spring Security’s filters.

	Note
	Refer to the relevant sections to see how to customize the defaults for both servlet and webflux based applications.

Spring Security provides a default set of security related HTTP response headers to provide secure defaults.

The default for Spring Security is to include the following headers:

Cache-Control: no-cache, no-store, max-age=0, must-revalidate
Pragma: no-cache
Expires: 0
X-Content-Type-Options: nosniff
Strict-Transport-Security: max-age=31536000 ; includeSubDomains
X-Frame-Options: DENY
X-XSS-Protection: 1; mode=block

	Note
	Strict-Transport-Security is only added on HTTPS requests

If the defaults do not meet your needs, you can easily remove, modify, or add headers from these defaults. For additional details on each of these headers, refer to the corresponding sections:

	Note
	Refer to the relevant sections to see how to customize the defaults for both servlet and webflux based applications.

Spring Security’s default is to disable caching to protect user’s content.

If a user authenticates to view sensitive information and then logs out, we don’t want a malicious user to be able to click the back button to view the sensitive information. The cache control headers that are sent by default are:

Cache-Control: no-cache, no-store, max-age=0, must-revalidate
Pragma: no-cache
Expires: 0

In order to be secure by default, Spring Security adds these headers by default. However, if your application provides it’s own cache control headers Spring Security will back out of the way. This allows for applications to ensure that static resources like CSS and JavaScript can be cached.

	Note
	Refer to the relevant sections to see how to customize the defaults for both servlet and webflux based applications.

Historically browsers, including Internet Explorer, would try to guess the content type of a request using content sniffing. This allowed browsers to improve the user experience by guessing the content type on resources that had not specified the content type. For example, if a browser encountered a JavaScript file that did not have the content type specified, it would be able to guess the content type and then execute it.

Note

There are many additional things one should do (i.e. only display the document in a distinct domain, ensure Content-Type header is set, sanitize the document, etc) when allowing content to be uploaded. However, these measures are out of the scope of what Spring Security provides. It is also important to point out when disabling content sniffing, you must specify the content type in order for things to work properly.

The problem with content sniffing is that this allowed malicious users to use polyglots (i.e. a file that is valid as multiple content types) to execute XSS attacks. For example, some sites may allow users to submit a valid postscript document to a website and view it. A malicious user might create a postscript document that is also a valid JavaScript file and execute a XSS attack with it.

Spring Security disables content sniffing by default by adding the following header to HTTP responses:

X-Content-Type-Options: nosniff

	Note
	Refer to the relevant sections to see how to customize the defaults for both servlet and webflux based applications.

When you type in your bank’s website, do you enter mybank.example.com or do you enter https://mybank.example.com? If you omit the https protocol, you are potentially vulnerable to Man in the Middle attacks. Even if the website performs a redirect to https://mybank.example.com a malicious user could intercept the initial HTTP request and manipulate the response (i.e. redirect to https://mibank.example.com and steal their credentials).

Many users omit the https protocol and this is why HTTP Strict Transport Security (HSTS) was created. Once mybank.example.com is added as a HSTS host, a browser can know ahead of time that any request to mybank.example.com should be interpreted as https://mybank.example.com. This greatly reduces the possibility of a Man in the Middle attack occurring.

	Note
	In accordance with RFC6797, the HSTS header is only injected into HTTPS responses. In order for the browser to acknowledge the header, the browser must first trust the CA that signed the SSL certificate used to make the connection (not just the SSL certificate).

One way for a site to be marked as a HSTS host is to have the host preloaded into the browser. Another is to add the Strict-Transport-Security header to the response. For example, Spring Security’s default behavior is to add the following header which instructs the browser to treat the domain as an HSTS host for a year (there are approximately 31536000 seconds in a year):

Strict-Transport-Security: max-age=31536000 ; includeSubDomains ; preload

The optional includeSubDomains directive instructs the browser that subdomains (i.e. secure.mybank.example.com) should also be treated as an HSTS domain.

The optional preload directive instructs the browser that domain should be preloaded in browser as HSTS domain. For more details on HSTS preload please see https://hstspreload.org.

	Note
	In order to remain passive Spring Security still provides support for HPKP in servlet environments, but for the reasons listed above HPKP is no longer recommended by the security team.

HTTP Public Key Pinning (HPKP) specifies to a web client which public key to use with certain web server to prevent Man in the Middle (MITM) attacks with forged certificates. When used correctly, HPKP could add additional layers of protection against compromised certificates. However, due to the complexity of HPKP many experts no longer recommend using it and Chrome has even removed support for it.

For additional details around why HPKP is no longer recommended read Is HTTP Public Key Pinning Dead? and I’m giving up on HPKP.

	Note
	Refer to the relevant sections to see how to customize the defaults for both servlet and webflux based applications.

Allowing your website to be added to a frame can be a security issue. For example, using clever CSS styling users could be tricked into clicking on something that they were not intending (video demo). For example, a user that is logged into their bank might click a button that grants access to other users. This sort of attack is known as Clickjacking.

	Note
	Another modern approach to dealing with clickjacking is to use the section called “Content Security Policy (CSP)”.

There are a number ways to mitigate clickjacking attacks. For example, to protect legacy browsers from clickjacking attacks you can use frame breaking code. While not perfect, the frame breaking code is the best you can do for the legacy browsers.

A more modern approach to address clickjacking is to use X-Frame-Options header. By default Spring Security disables rendering pages within an iframe using with the following header:

X-Frame-Options: DENY

	Note
	Refer to the relevant sections to see how to customize the defaults for both servlet and webflux based applications.

Some browsers have built in support for filtering out reflected XSS attacks. This is by no means foolproof, but does assist in XSS protection.

The filtering is typically enabled by default, so adding the header typically just ensures it is enabled and instructs the browser what to do when a XSS attack is detected. For example, the filter might try to change the content in the least invasive way to still render everything. At times, this type of replacement can become a XSS vulnerability in itself. Instead, it is best to block the content rather than attempt to fix it. By default Spring Security blocks the content using the following header:

X-XSS-Protection: 1; mode=block

	Note
	Refer to the relevant sections to see how to configure both servlet and webflux based applications.

Content Security Policy (CSP) is a mechanism that web applications can leverage to mitigate content injection vulnerabilities, such as cross-site scripting (XSS). CSP is a declarative policy that provides a facility for web application authors to declare and ultimately inform the client (user-agent) about the sources from which the web application expects to load resources.

	Note
	Content Security Policy is not intended to solve all content injection vulnerabilities. Instead, CSP can be leveraged to help reduce the harm caused by content injection attacks. As a first line of defense, web application authors should validate their input and encode their output.

A web application may employ the use of CSP by including one of the following HTTP headers in the response:

Each of these headers are used as a mechanism to deliver a security policy to the client. A security policy contains a set of security policy directives, each responsible for declaring the restrictions for a particular resource representation.

For example, a web application can declare that it expects to load scripts from specific, trusted sources, by including the following header in the response:

Content-Security-Policy: script-src https://trustedscripts.example.com

An attempt to load a script from another source other than what is declared in the script-src directive will be blocked by the user-agent. Additionally, if the report-uri directive is declared in the security policy, then the violation will be reported by the user-agent to the declared URL.

For example, if a web application violates the declared security policy, the following response header will instruct the user-agent to send violation reports to the URL specified in the policy’s report-uri directive.

Content-Security-Policy: script-src https://trustedscripts.example.com; report-uri /csp-report-endpoint/

Violation reports are standard JSON structures that can be captured either by the web application’s own API or by a publicly hosted CSP violation reporting service, such as, https://report-uri.io/.

The Content-Security-Policy-Report-Only header provides the capability for web application authors and administrators to monitor security policies, rather than enforce them. This header is typically used when experimenting and/or developing security policies for a site. When a policy is deemed effective, it can be enforced by using the Content-Security-Policy header field instead.

Given the following response header, the policy declares that scripts may be loaded from one of two possible sources.

Content-Security-Policy-Report-Only: script-src 'self' https://trustedscripts.example.com; report-uri /csp-report-endpoint/

If the site violates this policy, by attempting to load a script from evil.com, the user-agent will send a violation report to the declared URL specified by the report-uri directive, but still allow the violating resource to load nevertheless.

Applying Content Security Policy to a web application is often a non-trivial undertaking. The following resources may provide further assistance in developing effective security policies for your site.

An Introduction to Content Security Policy

CSP Guide - Mozilla Developer Network

W3C Candidate Recommendation

	Note
	Refer to the relevant sections to see how to configure both servlet and webflux based applications.

Referrer Policy is a mechanism that web applications can leverage to manage the referrer field, which contains the last page the user was on.

Spring Security’s approach is to use Referrer Policy header, which provides different policies:

Referrer-Policy: same-origin

The Referrer-Policy response header instructs the browser to let the destination knows the source where the user was previously.

	Note
	Refer to the relevant sections to see how to configure both servlet and webflux based applications.

Feature Policy is a mechanism that allows web developers to selectively enable, disable, and modify the behavior of certain APIs and web features in the browser.

Feature-Policy: geolocation 'self'

With Feature Policy, developers can opt-in to a set of "policies" for the browser to enforce on specific features used throughout your site. These policies restrict what APIs the site can access or modify the browser’s default behavior for certain features.

	Note
	Refer to the relevant sections to see how to configure both servlet and webflux based applications.

Clear Site Data is a mechanism by which any browser-side data - cookies, local storage, and the like - can be removed when an HTTP response contains this header:

Clear-Site-Data: "cache", "cookies", "storage", "executionContexts"

This is a nice clean-up action to perform on logout.

	Note
	Refer to the relevant sections to see how to configure both servlet based applications.

Spring Security has mechanisms to make it convenient to add the more common security headers to your application. However, it also provides hooks to enable adding custom headers.

When a client uses HTTP, Spring Security can be configured to redirect to HTTPS both Servlet and WebFlux environments.

Spring Security provides support for Strict Transport Security and enables it by default.

When using a proxy server it is important to ensure that you have configured your application properly. For example, many applications will have a load balancer that responds to request for https://example.com/ by forwarding the request to an application server at https://192.168.1:8080 Without proper configuration, the application server will not know that the load balancer exists and treat the request as though https://192.168.1:8080 was requested by the client.

To fix this you can use RFC 7239 to specify that a load balancer is being used. To make the application aware of this, you need to either configure your application server aware of the X-Forwarded headers. For example Tomcat uses the RemoteIpValve and Jetty uses ForwardedRequestCustomizer. Alternatively, Spring users can leverage ForwardedHeaderFilter.

Spring Boot users may use the server.use-forward-headers property to configure the application. See the Spring Boot documentation for further details.