[[webflux-csrf]]
= Cross Site Request Forgery (CSRF) for WebFlux Environments

This section discusses Spring Security's xref:features/exploits/csrf.adoc#csrf[Cross Site Request Forgery (CSRF)] support for WebFlux environments.

[[webflux-csrf-using]]
== Using Spring Security CSRF Protection
The steps to using Spring Security's CSRF protection are outlined below:

* <<webflux-csrf-idempotent,Use proper HTTP verbs>>
* <<webflux-csrf-configure,Configure CSRF Protection>>
* <<webflux-csrf-include,Include the CSRF Token>>

[[webflux-csrf-idempotent]]
=== Use Proper HTTP Verbs
The first step to protecting against CSRF attacks is to ensure your website uses proper HTTP verbs.
This is covered in detail in xref:features/exploits/csrf.adoc#csrf-protection-idempotent[Safe Methods Must be Idempotent].

[[webflux-csrf-configure]]
=== Configure CSRF Protection
The next step is to configure Spring Security's CSRF protection within your application.
By default, Spring Security's CSRF protection is enabled, but you may need to customize the configuration.
The next few subsections cover a few common customizations.

[[webflux-csrf-configure-custom-repository]]
==== Custom CsrfTokenRepository

By default, Spring Security stores the expected CSRF token in the `WebSession` by using `WebSessionServerCsrfTokenRepository`.
Sometimes, you may need to configure a custom `ServerCsrfTokenRepository`.
For example, you may want to persist the `CsrfToken` in a cookie to <<webflux-csrf-include-ajax-auto,support a JavaScript-based application>>.

By default, the `CookieServerCsrfTokenRepository` writes to a cookie named `XSRF-TOKEN` and read its from a header named `X-XSRF-TOKEN` or the HTTP `_csrf` parameter.
These defaults come from https://docs.angularjs.org/api/ng/service/$http#cross-site-request-forgery-xsrf-protection[AngularJS]

You can configure `CookieServerCsrfTokenRepository` in Java Configuration:

.Store CSRF Token in a Cookie
====
.Java
[source,java,role="primary"]
-----
@Bean
public SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.csrf(csrf -> csrf.csrfTokenRepository(CookieServerCsrfTokenRepository.withHttpOnlyFalse()))
	return http.build();
}
-----

.Kotlin
[source,kotlin,role="secondary"]
-----
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        // ...
        csrf {
            csrfTokenRepository = CookieServerCsrfTokenRepository.withHttpOnlyFalse()
        }
    }
}
-----
====

[NOTE]
====
The preceding sample explicitly sets `cookieHttpOnly=false`.
This is necessary to let JavaScript (in this case, AngularJS) to read it.
If you do not need the ability to read the cookie with JavaScript directly, we recommend to omitting `cookieHttpOnly=false` (by using `new CookieServerCsrfTokenRepository()` instead) to improve security.
====

[[webflux-csrf-configure-disable]]
==== Disable CSRF Protection
By default, CSRF protection is enabled.
However, you can disable CSRF protection if it xref:features/exploits/csrf.adoc#csrf-when[makes sense for your application].

The Java configuration below will disable CSRF protection.

.Disable CSRF Configuration
====
.Java
[source,java,role="primary"]
----
@Bean
public SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.csrf(csrf -> csrf.disable()))
	return http.build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
-----
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        // ...
        csrf {
            disable()
        }
    }
}
-----
====

[[webflux-csrf-include]]
=== Include the CSRF Token

For the xref:features/exploits/csrf.adoc#csrf-protection-stp[synchronizer token pattern] to protect against CSRF attacks, we must include the actual CSRF token in the HTTP request.
It must be included in a part of the request (a form parameter, an HTTP header, or other option) that is not automatically included in the HTTP request by the browser.

Spring Security's https://docs.spring.io/spring-security/site/docs/current/api/org/springframework/security/web/server/csrf/CsrfWebFilter.html[`CsrfWebFilter`] exposes a https://docs.spring.io/spring-security/site/docs/current/api/org/springframework/security/web/csrf/CsrfToken.html[`Mono<CsrfToken>`] as a `ServerWebExchange` attribute named `org.springframework.security.web.server.csrf.CsrfToken`.
This means that any view technology can access the `Mono<CsrfToken>` to expose the expected token as either a <<webflux-csrf-include-form-attr,form>> or a <<webflux-csrf-include-ajax-meta,meta tag>>.

[[webflux-csrf-include-subscribe]]
If your view technology does not provide a simple way to subscribe to the `Mono<CsrfToken>`, a common pattern is to use Spring's `@ControllerAdvice` to expose the `CsrfToken` directly.
The following example places the `CsrfToken` on the default attribute name (`_csrf`) used by Spring Security's <<webflux-csrf-include-form-auto,CsrfRequestDataValueProcessor>> to automatically include the CSRF token as a hidden input:

.`CsrfToken` as `@ModelAttribute`
====
.Java
[source,java,role="primary"]
----
@ControllerAdvice
public class SecurityControllerAdvice {
	@ModelAttribute
	Mono<CsrfToken> csrfToken(ServerWebExchange exchange) {
		Mono<CsrfToken> csrfToken = exchange.getAttribute(CsrfToken.class.getName());
		return csrfToken.doOnSuccess(token -> exchange.getAttributes()
				.put(CsrfRequestDataValueProcessor.DEFAULT_CSRF_ATTR_NAME, token));
	}
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@ControllerAdvice
class SecurityControllerAdvice {
    @ModelAttribute
    fun csrfToken(exchange: ServerWebExchange): Mono<CsrfToken> {
        val csrfToken: Mono<CsrfToken>? = exchange.getAttribute(CsrfToken::class.java.name)
        return csrfToken!!.doOnSuccess { token ->
            exchange.attributes[CsrfRequestDataValueProcessor.DEFAULT_CSRF_ATTR_NAME] = token
        }
    }
}
----
====

Fortunately, Thymeleaf provides <<webflux-csrf-include-form-auto,integration>> that works without any additional work.

[[webflux-csrf-include-form]]
==== Form URL Encoded
To post an HTML form, the CSRF token must be included in the form as a hidden input.
The following example shows what the rendered HTML might look like:

.CSRF Token HTML
====
[source,html]
----
<input type="hidden"
	name="_csrf"
	value="4bfd1575-3ad1-4d21-96c7-4ef2d9f86721"/>
----
====

Next, we discuss various ways of including the CSRF token in a form as a hidden input.

[[webflux-csrf-include-form-auto]]
===== Automatic CSRF Token Inclusion

Spring Security's CSRF support provides integration with Spring's https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/web/reactive/result/view/RequestDataValueProcessor.html[`RequestDataValueProcessor`] through its https://docs.spring.io/spring-security/site/docs/current/api/org/springframework/security/web/reactive/result/view/CsrfRequestDataValueProcessor.html[`CsrfRequestDataValueProcessor`].
For `CsrfRequestDataValueProcessor` to work, the `Mono<CsrfToken>` must be subscribed to and the `CsrfToken` must be <<webflux-csrf-include-subscribe,exposed as an attribute>> that matches https://docs.spring.io/spring-security/site/docs/current/api/org/springframework/security/web/reactive/result/view/CsrfRequestDataValueProcessor.html#DEFAULT_CSRF_ATTR_NAME[`DEFAULT_CSRF_ATTR_NAME`].

Fortunately, Thymeleaf https://www.thymeleaf.org/doc/tutorials/2.1/thymeleafspring.html#integration-with-requestdatavalueprocessor[takes care of all the boilerplate] for you by integrating with `RequestDataValueProcessor` to ensure that forms that have an unsafe HTTP method (POST) automatically include the actual CSRF token.

[[webflux-csrf-include-form-attr]]
===== CsrfToken Request Attribute

If the <<webflux-csrf-include,other options>> for including the actual CSRF token in the request do not work, you can take advantage of the fact that the `Mono<CsrfToken>` <<webflux-csrf-include,is exposed>> as a `ServerWebExchange` attribute named `org.springframework.security.web.server.csrf.CsrfToken`.

The following Thymeleaf sample assumes that you <<webflux-csrf-include-subscribe,expose>> the `CsrfToken` on an attribute named `_csrf`:

.CSRF Token in Form with Request Attribute
====
[source,html]
----
<form th:action="@{/logout}"
	method="post">
<input type="submit"
	value="Log out" />
<input type="hidden"
	th:name="${_csrf.parameterName}"
	th:value="${_csrf.token}"/>
</form>
----
====

[[webflux-csrf-include-ajax]]
==== Ajax and JSON Requests
If you use JSON, you cannot submit the CSRF token within an HTTP parameter.
Instead, you can submit the token within a HTTP header.

In the following sections, we discuss various ways of including the CSRF token as an HTTP request header in JavaScript-based applications.

[[webflux-csrf-include-ajax-auto]]
===== Automatic Inclusion

You can <<webflux-csrf-configure-custom-repository,configure>> Spring Security to store the expected CSRF token in a cookie.
By storing the expected CSRF in a cookie, JavaScript frameworks, such as https://docs.angularjs.org/api/ng/service/$http#cross-site-request-forgery-xsrf-protection[AngularJS], automatically include the actual CSRF token in the HTTP request headers.

[[webflux-csrf-include-ajax-meta]]
===== Meta Tags

An alternative pattern to <<webflux-csrf-include-form-auto,exposing the CSRF in a cookie>> is to include the CSRF token within your `meta` tags.
The HTML might look something like this:

.CSRF meta tag HTML
====
[source,html]
----
<html>
<head>
	<meta name="_csrf" content="4bfd1575-3ad1-4d21-96c7-4ef2d9f86721"/>
	<meta name="_csrf_header" content="X-CSRF-TOKEN"/>
	<!-- ... -->
</head>
<!-- ... -->
----
====

Once the meta tags contain the CSRF token, the JavaScript code can read the meta tags and include the CSRF token as a header.
If you use jQuery, you could read the meta tags with the following code:

.AJAX send CSRF Token
====
[source,javascript]
----
$(function () {
	var token = $("meta[name='_csrf']").attr("content");
	var header = $("meta[name='_csrf_header']").attr("content");
	$(document).ajaxSend(function(e, xhr, options) {
		xhr.setRequestHeader(header, token);
	});
});
----
====

The following sample assumes that you <<webflux-csrf-include-subscribe,expose>> the `CsrfToken` on an attribute named `_csrf`.
The following example does this with Thymeleaf:

.CSRF meta tag JSP
====
[source,html]
----
<html>
<head>
	<meta name="_csrf" th:content="${_csrf.token}"/>
	<!-- default header name is X-CSRF-TOKEN -->
	<meta name="_csrf_header" th:content="${_csrf.headerName}"/>
	<!-- ... -->
</head>
<!-- ... -->
----
====

[[webflux-csrf-considerations]]
== CSRF Considerations
There are a few special considerations to consider when implementing protection against CSRF attacks.
This section discusses those considerations as it pertains to WebFlux environments.
See xref:features/exploits/csrf.adoc#csrf-considerations[CSRF Considerations] for a more general discussion.


[[webflux-considerations-csrf-login]]
=== Logging In

You should xref:features/exploits/csrf.adoc#csrf-considerations-login[require CSRF for login] requests to protect against forged login attempts.
Spring Security's WebFlux support automatically does this.

[[webflux-considerations-csrf-logout]]
=== Logging Out

You should xref:features/exploits/csrf.adoc#csrf-considerations-logout[require CSRF for logout] requests to protect against forging logout attempts.
By default, Spring Security's `LogoutWebFilter` only processes only HTTP post requests.
This ensures that logout requires a CSRF token and that a malicious user cannot forcibly log out your users.

The easiest approach is to use a form to log out.
If you really want a link, you can use JavaScript to have the link perform a POST (maybe on a hidden form).
For browsers with JavaScript that is disabled, you can optionally have the link take the user to a logout confirmation page that performs the POST.

If you really want to use HTTP GET with logout, you can do so, but remember that doing so is generally not recommended.
For example, the following Java Configuration logs out when the `/logout` URL is requested with any HTTP method:

// FIXME: This should be a link to log out documentation

.Log out with HTTP GET
====
.Java
[source,java,role="primary"]
----
@Bean
public SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.logout(logout -> logout.requiresLogout(new PathPatternParserServerWebExchangeMatcher("/logout")))
	return http.build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        // ...
        logout {
            requiresLogout = PathPatternParserServerWebExchangeMatcher("/logout")
        }
    }
}
----
====


[[webflux-considerations-csrf-timeouts]]
=== CSRF and Session Timeouts

By default, Spring Security stores the CSRF token in the `WebSession`.
This arrangement can lead to a situation where the session expires, which means that there is not an expected CSRF token to validate against.

We have already discussed xref:features/exploits/csrf.adoc#csrf-considerations-login[general solutions] to session timeouts.
This section discusses the specifics of CSRF timeouts as it pertains to the WebFlux support.

You can change storage of the expected CSRF token to be in a cookie.
For details, see the <<webflux-csrf-configure-custom-repository>> section.

// FIXME: We should add a custom AccessDeniedHandler section in the reference and update the earlier links

// FIXME: We need a WebFlux multipart body vs action story. WebFlux always has multipart enabled.
[[webflux-csrf-considerations-multipart]]
=== Multipart (file upload)
We have xref:features/exploits/csrf.adoc#csrf-considerations-multipart[already discussed] how protecting multipart requests (file uploads) from CSRF attacks causes a https://en.wikipedia.org/wiki/Chicken_or_the_egg[chicken and the egg] problem.
This section discusses how to implement placing the CSRF token in the <<webflux-csrf-considerations-multipart-body,body>> and <<webflux-csrf-considerations-multipart-url,url>> within a WebFlux application.

[NOTE]
====
For more information about using multipart forms with Spring, see the https://docs.spring.io/spring/docs/5.2.x/spring-framework-reference/web-reactive.html#webflux-multipart[Multipart Data] section of the Spring reference.
====

[[webflux-csrf-considerations-multipart-body]]
==== Place CSRF Token in the Body

We have xref:features/exploits/csrf.adoc#csrf-considerations-multipart[already discussed] the trade-offs of placing the CSRF token in the body.

In a WebFlux application, you can do so with the following configuration:

.Enable obtaining CSRF token from multipart/form-data
====
.Java
[source,java,role="primary"]
----
@Bean
public SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.csrf(csrf -> csrf.tokenFromMultipartDataEnabled(true))
	return http.build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
		// ...
        csrf {
            tokenFromMultipartDataEnabled = true
        }
    }
}
----
====

[[webflux-csrf-considerations-multipart-url]]
==== Include CSRF Token in URL

We have xref:features/exploits/csrf.adoc#csrf-considerations-multipart[already discussed] the trade-offs of placing the CSRF token in the URL.
Since the `CsrfToken` is exposed as an `ServerHttpRequest` <<webflux-csrf-include,request attribute>>, we can use that to create an `action` with the CSRF token in it.
An example with Thymeleaf is shown below:

.CSRF Token in Action
====
[source,html]
----
<form method="post"
	th:action="@{/upload(${_csrf.parameterName}=${_csrf.token})}"
	enctype="multipart/form-data">
----
====

[[webflux-csrf-considerations-override-method]]
=== HiddenHttpMethodFilter
We have xref:features/exploits/csrf.adoc#csrf-considerations-override-method[already discussed] overriding the HTTP method.

In a Spring WebFlux application, overriding the HTTP method is done by using https://docs.spring.io/spring-framework/docs/5.2.x/javadoc-api/org/springframework/web/filter/reactive/HiddenHttpMethodFilter.html[`HiddenHttpMethodFilter`].
