
[[webflux-headers]]
 Security HTTP Response Headers
This section discusses Spring Security's support for adding various security headers to the response of WebFlux.

= Default Security Headers
Spring Security allows users to easily inject the default security headers to assist in protecting their application.
The default for Spring Security is to include the following headers:

[source,http]
----
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

For additional details on each of these headers, refer to the corresponding sections:

* <<webflux-headers-cache-control,Cache Control>>
* <<webflux-headers-content-type-options,Content Type Options>>
* <<webflux-headers-hsts,HTTP Strict Transport Security>>
* <<webflux-headers-frame-options,X-Frame-Options>>
* <<webflux-headers-xss-protection,X-XSS-Protection>>

While each of these headers are considered best practice, it should be noted that not all clients utilize the headers, so additional testing is encouraged.

You can customize specific headers.
For example, assume that want your HTTP response headers to look like the following:

[source,http]
----
Cache-Control: no-cache, no-store, max-age=0, must-revalidate
Pragma: no-cache
Expires: 0
X-Content-Type-Options: nosniff
X-Frame-Options: SAMEORIGIN
X-XSS-Protection: 1; mode=block
----

Specifically, you want all of the default headers with the following customizations:

* <<webflux-headers-frame-options,X-Frame-Options>> to allow any request from same domain
* <<webflux-headers-hsts,HTTP Strict Transport Security (HSTS)>> will not be added to the response

You can easily do this with the following Java Configuration:

[source,java]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.headers(headers ->
			headers
				.hsts(hsts ->
					hsts
						.disable()
				)
				.frameOptions(frameOptions ->
					frameOptions
						.mode(Mode.SAMEORIGIN)
				)
		);
	return http.build();
}
----


If you do not want the defaults to be added and want explicit control over what should be used, you can disable the defaults.
An example for both Java and XML based configuration is provided below:

If necessary, you can disable all of the HTTP Security response headers with the following Java Configuration:

[source,java]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.headers(headers ->
			headers
				.disable()
		);
	return http.build();
}
----

[[webflux-headers-cache-control]]
== Cache Control
In the past Spring Security required you to provide your own cache control for your web application.
This seemed reasonable at the time, but browser caches have evolved to include caches for secure connections as well.
This means that a user may view an authenticated page, log out, and then a malicious user can use the browser history to view the cached page.
To help mitigate this Spring Security has added cache control support which will insert the following headers into you response by default.

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


If you actually want to cache specific responses, your application can selectively set the cache control headers to override the header set by Spring Security.
This is useful to ensure things like CSS, JavaScript, and images are properly cached.

You can also disable cache control using the following Java Configuration:

[source,java]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.headers(headers ->
			headers
				.cache(cache -> cache.disable())
		);
	return http.build();
}
----

[[webflux-headers-content-type-options]]
== Content Type Options
Historically browsers, including Internet Explorer, would try to guess the content type of a request using https://en.wikipedia.org/wiki/Content_sniffing[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 http://webblaze.cs.berkeley.edu/papers/barth-caballero-song.pdf[postscript document that is also a valid JavaScript file] and execute a XSS attack with it.

Content sniffing can be disabled by adding the following header to our response:

[source]
----
X-Content-Type-Options: nosniff
----

Just as with the cache control element, the nosniff directive is added by default.
However, if need to disable the header, the following may be used:

[source,java]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.headers(headers ->
			headers
				.contentTypeOptions(contentTypeOptions -> contentTypeOptions.disable())
		);
	return http.build();
}
----

[[webflux-headers-hsts]]
== HTTP Strict Transport Security (HSTS)
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 https://en.wikipedia.org/wiki/Man-in-the-middle_attack[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 https://tools.ietf.org/html/rfc6797[HTTP Strict Transport Security (HSTS)] was created.
Once mybank.example.com is added as a https://tools.ietf.org/html/rfc6797#section-5.1[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 https://tools.ietf.org/html/rfc6797#section-7.2[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 the following would instruct the browser to treat the domain as an HSTS host for a year (there are approximately 31536000 seconds in a year):

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

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

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

As with the other headers, Spring Security adds HSTS by default.
You can customize HSTS headers with Java Configuration:

[source,java]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.headers(headers ->
			headers
				.hsts(hsts ->
					hsts
						.includeSubdomains(true)
						.preload(true)
						.maxAge(Duration.ofDays(365))
				)
		);
	return http.build();
}
----


[[webflux-headers-frame-options]]
== X-Frame-Options
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 (https://www.youtube.com/watch?v=3mk0RySeNsU[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 https://en.wikipedia.org/wiki/Clickjacking[Clickjacking].

[NOTE]
==
Another modern approach to dealing with clickjacking is to use <<webflux-headers-csp>>.
==

There are a number ways to mitigate clickjacking attacks.
For example, to protect legacy browsers from clickjacking attacks you can use https://www.owasp.org/index.php/Clickjacking_Defense_Cheat_Sheet#Best-for-now_Legacy_Browser_Frame_Breaking_Script[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 https://developer.mozilla.org/en-US/docs/HTTP/X-Frame-Options[X-Frame-Options] header:

[source]
----
X-Frame-Options: DENY
----

The X-Frame-Options response header instructs the browser to prevent any site with this header in the response from being rendered within a frame.
By default, Spring Security disables rendering within an iframe.

You can customize X-Frame-Options with Java Configuration using the following:

[source,java]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.headers(headers ->
			headers
				.frameOptions(frameOptions ->
					frameOptions
						.mode(SAMEORIGIN)
				)
		);
	return http.build();
}
----

[[webflux-headers-xss-protection]]
== X-XSS-Protection
Some browsers have built in support for filtering out https://www.owasp.org/index.php/Testing_for_Reflected_Cross_site_scripting_(OWASP-DV-001)[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 https://hackademix.net/2009/11/21/ies-xss-filter-creates-xss-vulnerabilities/[XSS vulnerability in itself].
Instead, it is best to block the content rather than attempt to fix it.
To do this we can add the following header:

[source]
----
X-XSS-Protection: 1; mode=block
----

This header is included by default.
However, we can customize with Java Configuration with the following:

[source,java]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.headers(headers ->
			headers
				.xssProtection(xssProtection -> xssProtection.disable())
		);
	return http.build();
}
----

[[webflux-headers-csp]]
== Content Security Policy (CSP)

https://www.w3.org/TR/CSP2/[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:

* *_Content-Security-Policy_*
* *_Content-Security-Policy-Report-Only_*

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_* (for example, _script-src_ and _object-src_), 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:

[source]
----
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 https://www.w3.org/TR/CSP2/#directive-report-uri[*_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.

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

https://www.w3.org/TR/CSP2/#violation-reports[*_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/[*_REPORT-URI_*].

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.

[source]
----
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.

[[webflux-headers-csp-configure]]
=== Configuring Content Security Policy

It's important to note that Spring Security *_does not add_* Content Security Policy by default.
The web application author must declare the security policy(s) to enforce and/or monitor for the protected resources.

For example, given the following security policy:

[source]
----
script-src 'self' https://trustedscripts.example.com; object-src https://trustedplugins.example.com; report-uri /csp-report-endpoint/
----

You can enable the CSP header using Java configuration as shown below:

[source,java]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.headers(headers ->
			headers
				.contentSecurityPolicy(contentSecurityPolicy ->
					contentSecurityPolicy
						.policyDirectives("script-src 'self' https://trustedscripts.example.com; object-src https://trustedplugins.example.com; report-uri /csp-report-endpoint/")
				)
		);
	return http.build();
}
----

To enable the CSP _'report-only'_ header, provide the following Java configuration:

[source,java]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.headers(headers ->
			headers
				.contentSecurityPolicy(contentSecurityPolicy ->
					contentSecurityPolicy
						.policyDirectives("script-src 'self' https://trustedscripts.example.com; object-src https://trustedplugins.example.com; report-uri /csp-report-endpoint/")
						.reportOnly()
				)
		);
	return http.build();
}
----

[[webflux-headers-csp-links]]
=== Additional Resources

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.

https://www.html5rocks.com/en/tutorials/security/content-security-policy/[An Introduction to Content Security Policy]

https://developer.mozilla.org/en-US/docs/Web/Security/CSP[CSP Guide - Mozilla Developer Network]

https://www.w3.org/TR/CSP2/[W3C Candidate Recommendation]

[[webflux-headers-referrer]]
== Referrer Policy

https://www.w3.org/TR/referrer-policy[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 https://www.w3.org/TR/referrer-policy/[Referrer Policy] header, which provides different https://www.w3.org/TR/referrer-policy/#referrer-policies[policies]:

[source]
----
Referrer-Policy: same-origin
----

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

[[webflux-headers-referrer-configure]]
=== Configuring Referrer Policy

Spring Security *_doesn't add_* Referrer Policy header by default.

You can enable the Referrer-Policy header using Java configuration as shown below:

[source,java]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.headers(headers ->
			headers
				.referrerPolicy(referrerPolicy ->
					referrerPolicy
						.policy(ReferrerPolicy.SAME_ORIGIN)
				)
		);
	return http.build();
}
----


[[webflux-headers-feature]]
== Feature Policy

https://wicg.github.io/feature-policy/[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.

[source]
----
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.

[[webflux-headers-feature-configure]]
=== Configuring Feature Policy

Spring Security *_doesn't add_* Feature Policy header by default.

You can enable the Feature-Policy header using Java configuration as shown below:

[source,java]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		// ...
		.headers(headers ->
			headers
				.featurePolicy("geolocation 'self'")
		);
	return http.build();
}
----
