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Cross Site Request Forgery (CSRF) for WebFlux Environments

This section discusses Spring Security’s Cross Site Request Forgery (CSRF) support for WebFlux environments.

Using Spring Security CSRF Protection

The steps to using Spring Security’s CSRF protection are outlined below:

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 Safe Methods Must be Read-only.

Configure CSRF Protection

The next step is to configure Spring Security’s CSRF protection within your application. Spring Security’s CSRF protection is enabled by default, but you may need to customize the configuration. Below are a few common customizations.

Custom CsrfTokenRepository

By default Spring Security stores the expected CSRF token in the WebSession using WebSessionServerCsrfTokenRepository. There can be cases where users will want to configure a custom ServerCsrfTokenRepository. For example, it might be desirable to persist the CsrfToken in a cookie to support a JavaScript based application.

By default the CookieServerCsrfTokenRepository will write to a cookie named XSRF-TOKEN and read it from a header named X-XSRF-TOKEN or the HTTP parameter _csrf. These defaults come from AngularJS

You can configure CookieServerCsrfTokenRepository in Java Configuration using:

Store CSRF Token in a Cookie

  • Java

  • Kotlin

@Bean
public SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.csrf(csrf -> csrf.csrfTokenRepository(CookieServerCsrfTokenRepository.withHttpOnlyFalse()))
	return http.build();
}
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        // ...
        csrf {
            csrfTokenRepository = CookieServerCsrfTokenRepository.withHttpOnlyFalse()
        }
    }
}

The sample explicitly sets cookieHttpOnly=false. This is necessary to allow JavaScript (i.e. AngularJS) to read it. If you do not need the ability to read the cookie with JavaScript directly, it is recommended to omit cookieHttpOnly=false (by using new CookieServerCsrfTokenRepository() instead) to improve security.

Disable CSRF Protection

CSRF protection is enabled by default. However, it is simple to disable CSRF protection if it makes sense for your application.

The Java configuration below will disable CSRF protection.

Disable CSRF Configuration

  • Java

  • Kotlin

@Bean
public SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.csrf(csrf -> csrf.disable()))
	return http.build();
}
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        // ...
        csrf {
            disable()
        }
    }
}

Configure ServerCsrfTokenRequestHandler

Spring Security’s CsrfWebFilter exposes a Mono<CsrfToken> as a ServerWebExchange attribute named org.springframework.security.web.server.csrf.CsrfToken with the help of a ServerCsrfTokenRequestHandler. The default implementation is ServerCsrfTokenRequestAttributeHandler.

An alternate implementation XorServerCsrfTokenRequestAttributeHandler is available to provide protection for BREACH (see gh-4001).

You can configure XorServerCsrfTokenRequestAttributeHandler using the following Java configuration:

Configure BREACH protection

  • Java

  • Kotlin

@Bean
public SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.csrf(csrf -> csrf
			.csrfTokenRequestHandler(new XorServerCsrfTokenRequestAttributeHandler())
		)
	return http.build();
}
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        // ...
        csrf {
            csrfTokenRequestHandler = XorServerCsrfTokenRequestAttributeHandler()
        }
    }
}

Include the CSRF Token

In order for the synchronizer token pattern to protect against CSRF attacks, we must include the actual CSRF token in the HTTP request. This must be included in a part of the request (i.e. form parameter, HTTP header, etc) that is not automatically included in the HTTP request by the browser.

We’ve seen that the Mono<CsrfToken> is exposed as a ServerWebExchange attribute. This means that any view technology can access the Mono<CsrfToken> to expose the expected token as either a form or meta tag.

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. For example, the following code will place the CsrfToken on the default attribute name (_csrf) used by Spring Security’s CsrfRequestDataValueProcessor to automatically include the CSRF token as a hidden input.

CsrfToken as @ModelAttribute

  • Java

  • Kotlin

@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));
	}
}
@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 integration that works without any additional work.

Form URL Encoded

In order to post an HTML form the CSRF token must be included in the form as a hidden input. For example, the rendered HTML might look like:

CSRF Token HTML
<input type="hidden"
	name="_csrf"
	value="4bfd1575-3ad1-4d21-96c7-4ef2d9f86721"/>

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

Automatic CSRF Token Inclusion

Spring Security’s CSRF support provides integration with Spring’s RequestDataValueProcessor via its CsrfRequestDataValueProcessor. In order for CsrfRequestDataValueProcessor to work, the Mono<CsrfToken> must be subscribed to and the CsrfToken must be exposed as an attribute that matches DEFAULT_CSRF_ATTR_NAME.

Fortunately, Thymeleaf provides support to take care of all the boilerplate for you by integrating with RequestDataValueProcessor to ensure that forms that have an unsafe HTTP method (i.e. post) will automatically include the actual CSRF token.

CsrfToken Request Attribute

If the 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> is exposed as a ServerWebExchange attribute named org.springframework.security.web.server.csrf.CsrfToken.

The Thymeleaf sample below assumes that you expose the CsrfToken on an attribute named _csrf.

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

Ajax and JSON Requests

If you are using JSON, then it is not possible to submit the CSRF token within an HTTP parameter. Instead you can submit the token within a HTTP header.

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

Automatic Inclusion

Spring Security can easily be configured to store the expected CSRF token in a cookie. By storing the expected CSRF in a cookie, JavaScript frameworks like AngularJS will automatically include the actual CSRF token in the HTTP request headers.

Meta tags

An alternative pattern to 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
<html>
<head>
	<meta name="_csrf" content="4bfd1575-3ad1-4d21-96c7-4ef2d9f86721"/>
	<meta name="_csrf_header" content="X-CSRF-TOKEN"/>
	<!-- ... -->
</head>
<!-- ... -->

Once the meta tags contained the CSRF token, the JavaScript code would read the meta tags and include the CSRF token as a header. If you were using jQuery, this could be done with the following:

AJAX send CSRF Token
$(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 sample below assumes that you expose the CsrfToken on an attribute named _csrf. An example of doing this with Thymeleaf is shown below:

CSRF meta tag JSP
<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>
<!-- ... -->

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. Refer to CSRF Considerations for a more general discussion.

Logging In

It is important to require CSRF for log in requests to protect against forging log in attempts. Spring Security’s WebFlux support does this out of the box.

Logging Out

It is important to require CSRF for log out requests to protect against forging log out attempts. By default Spring Security’s LogoutWebFilter only processes HTTP post requests. This ensures that log out 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 (i.e. maybe on a hidden form). For browsers with JavaScript that is disabled, you can optionally have the link take the user to a log out confirmation page that will perform the POST.

If you really want to use HTTP GET with logout you can do so, but remember this is generally not recommended. For example, the following Java Configuration will perform logout with the URL /logout is requested with any HTTP method:

Log out with HTTP GET

  • Java

  • Kotlin

@Bean
public SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.logout(logout -> logout.requiresLogout(new PathPatternParserServerWebExchangeMatcher("/logout")))
	return http.build();
}
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        // ...
        logout {
            requiresLogout = PathPatternParserServerWebExchangeMatcher("/logout")
        }
    }
}

CSRF and Session Timeouts

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

We’ve already discussed general solutions to session timeouts. This section discusses the specifics of CSRF timeouts as it pertains to the WebFlux support.

It is simple to change storage of the expected CSRF token to be in a cookie. For details, refer to the Custom CsrfTokenRepository section.

Multipart (file upload)

We have already discussed how protecting multipart requests (file uploads) from CSRF attacks causes a chicken and the egg problem. This section discusses how to implement placing the CSRF token in the body and url within a WebFlux application.

More information about using multipart forms with Spring can be found within the Multipart Data section of the Spring reference.

Place CSRF Token in the Body

We have already discussed the trade-offs of placing the CSRF token in the body.

In a WebFlux application, this can be configured with the following configuration:

Enable obtaining CSRF token from multipart/form-data

  • Java

  • Kotlin

@Bean
public SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.csrf(csrf -> csrf.tokenFromMultipartDataEnabled(true))
	return http.build();
}
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
		// ...
        csrf {
            tokenFromMultipartDataEnabled = true
        }
    }
}

Include CSRF Token in URL

We have already discussed the trade-offs of placing the CSRF token in the URL. Since the CsrfToken is exposed as an ServerHttpRequest 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
<form method="post"
	th:action="@{/upload(${_csrf.parameterName}=${_csrf.token})}"
	enctype="multipart/form-data">

HiddenHttpMethodFilter

We have already discussed overriding the HTTP method.

In a Spring WebFlux application, overriding the HTTP method is done using HiddenHttpMethodFilter.