Add Firewall for WebFlux

Closes gh-15967

docs/modules/ROOT/nav.adoc

@@ -147,6 +147,7 @@
 *** xref:reactive/exploits/csrf.adoc[CSRF]
 *** xref:reactive/exploits/headers.adoc[Headers]
 *** xref:reactive/exploits/http.adoc[HTTP Requests]
+*** xref:reactive/exploits/firewall.adoc[]
 ** Integrations
 *** xref:reactive/integrations/cors.adoc[CORS]
 *** xref:reactive/integrations/rsocket.adoc[RSocket]

docs/modules/ROOT/pages/reactive/exploits/firewall.adoc

@@ -0,0 +1,202 @@
+[[webflux-serverwebexchangefirewall]]
+= ServerWebExchangeFirewall
+
+There are various ways a request can be created by malicious users that can exploit applications.
+Spring Security provides the `ServerWebExchangeFirewall` to allow rejecting requests that look malicious.
+The default implementation is `StrictServerWebExchangeFirewall` which rejects malicious requests.
+
+For example a request could contain path-traversal sequences (such as `/../`) or multiple forward slashes (`//`) that could also cause pattern-matches to fail.
+Some containers normalize these out before performing the servlet mapping, but others do not.
+To protect against issues like these, `WebFilterChainProxy` uses a `ServerWebExchangeFirewall` strategy to check and wrap the request.
+By default, un-normalized requests are automatically rejected, and path parameters are removed for matching purposes.
+(So, for example, an original request path of `/secure;hack=1/somefile.html;hack=2` is returned as `/secure/somefile.html`.)
+It is, therefore, essential that a `WebFilterChainProxy` is used.
+
+In practice, we recommend that you use method security at your service layer, to control access to your application, rather than rely entirely on the use of security constraints defined at the web-application level.
+URLs change, and it is difficult to take into account all the possible URLs that an application might support and how requests might be manipulated.
+You should restrict yourself to using a few simple patterns that are simple to understand.
+Always try to use a "`deny-by-default`" approach, where you have a catch-all wildcard (`/**` or `**`) defined last to deny access.
+
+Security defined at the service layer is much more robust and harder to bypass, so you should always take advantage of Spring Security's method security options.
+
+You can customize the `ServerWebExchangeFirewall` by exposing it as a Bean.
+
+.Allow Matrix Variables
+[tabs]
+======
+Java::
++
+[source,java,role="primary"]
+----
+@Bean
+public StrictServerWebExchangeFirewall httpFirewall() {
+    StrictServerWebExchangeFirewall firewall = new StrictServerWebExchangeFirewall();
+    firewall.setAllowSemicolon(true);
+    return firewall;
+}
+----
+
+Kotlin::
++
+[source,kotlin,role="secondary"]
+----
+@Bean
+fun httpFirewall(): StrictServerWebExchangeFirewall {
+    val firewall = StrictServerWebExchangeFirewall()
+    firewall.setAllowSemicolon(true)
+    return firewall
+}
+----
+======
+
+To protect against https://www.owasp.org/index.php/Cross_Site_Tracing[Cross Site Tracing (XST)] and https://www.owasp.org/index.php/Test_HTTP_Methods_(OTG-CONFIG-006)[HTTP Verb Tampering], the `StrictServerWebExchangeFirewall` provides an allowed list of valid HTTP methods that are allowed.
+The default valid methods are `DELETE`, `GET`, `HEAD`, `OPTIONS`, `PATCH`, `POST`, and `PUT`.
+If your application needs to modify the valid methods, you can configure a custom `StrictServerWebExchangeFirewall` bean.
+The following example allows only HTTP `GET` and `POST` methods:
+
+
+.Allow Only GET & POST
+[tabs]
+======
+Java::
++
+[source,java,role="primary"]
+----
+@Bean
+public StrictServerWebExchangeFirewall httpFirewall() {
+    StrictServerWebExchangeFirewall firewall = new StrictServerWebExchangeFirewall();
+    firewall.setAllowedHttpMethods(Arrays.asList("GET", "POST"));
+    return firewall;
+}
+----
+
+Kotlin::
++
+[source,kotlin,role="secondary"]
+----
+@Bean
+fun httpFirewall(): StrictServerWebExchangeFirewall {
+    val firewall = StrictServerWebExchangeFirewall()
+    firewall.setAllowedHttpMethods(listOf("GET", "POST"))
+    return firewall
+}
+----
+======
+
+If you must allow any HTTP method (not recommended), you can use `StrictServerWebExchangeFirewall.setUnsafeAllowAnyHttpMethod(true)`.
+Doing so entirely disables validation of the HTTP method.
+
+
+[[webflux-serverwebexchangefirewall-headers-parameters]]
+`StrictServerWebExchangeFirewall` also checks header names and values and parameter names.
+It requires that each character have a defined code point and not be a control character.
+
+This requirement can be relaxed or adjusted as necessary by using the following methods:
+
+* `StrictServerWebExchangeFirewall#setAllowedHeaderNames(Predicate)`
+* `StrictServerWebExchangeFirewall#setAllowedHeaderValues(Predicate)`
+* `StrictServerWebExchangeFirewall#setAllowedParameterNames(Predicate)`
+
+[NOTE]
+====
+Parameter values can be also controlled with `setAllowedParameterValues(Predicate)`.
+====
+
+For example, to switch off this check, you can wire your `StrictServerWebExchangeFirewall` with `Predicate` instances that always return `true`:
+
+.Allow Any Header Name, Header Value, and Parameter Name
+[tabs]
+======
+Java::
++
+[source,java,role="primary"]
+----
+@Bean
+public StrictServerWebExchangeFirewall httpFirewall() {
+    StrictServerWebExchangeFirewall firewall = new StrictServerWebExchangeFirewall();
+    firewall.setAllowedHeaderNames((header) -> true);
+    firewall.setAllowedHeaderValues((header) -> true);
+    firewall.setAllowedParameterNames((parameter) -> true);
+    return firewall;
+}
+----
+
+Kotlin::
++
+[source,kotlin,role="secondary"]
+----
+@Bean
+fun httpFirewall(): StrictServerWebExchangeFirewall {
+    val firewall = StrictServerWebExchangeFirewall()
+    firewall.setAllowedHeaderNames { true }
+    firewall.setAllowedHeaderValues { true }
+    firewall.setAllowedParameterNames { true }
+    return firewall
+}
+----
+======
+
+Alternatively, there might be a specific value that you need to allow.
+
+For example, iPhone Xʀ uses a `User-Agent` that includes a character that is not in the ISO-8859-1 charset.
+Due to this fact, some application servers parse this value into two separate characters, the latter being an undefined character.
+
+You can address this with the `setAllowedHeaderValues` method:
+
+.Allow Certain User Agents
+[tabs]
+======
+Java::
++
+[source,java,role="primary"]
+----
+@Bean
+public StrictServerWebExchangeFirewall httpFirewall() {
+    StrictServerWebExchangeFirewall firewall = new StrictServerWebExchangeFirewall();
+    Pattern allowed = Pattern.compile("[\\p{IsAssigned}&&[^\\p{IsControl}]]*");
+    Pattern userAgent = ...;
+    firewall.setAllowedHeaderValues((header) -> allowed.matcher(header).matches() || userAgent.matcher(header).matches());
+    return firewall;
+}
+----
+
+Kotlin::
++
+[source,kotlin,role="secondary"]
+----
+@Bean
+fun httpFirewall(): StrictServerWebExchangeFirewall {
+    val firewall = StrictServerWebExchangeFirewall()
+    val allowed = Pattern.compile("[\\p{IsAssigned}&&[^\\p{IsControl}]]*")
+    val userAgent = Pattern.compile(...)
+    firewall.setAllowedHeaderValues { allowed.matcher(it).matches() || userAgent.matcher(it).matches() }
+    return firewall
+}
+----
+======
+
+In the case of header values, you may instead consider parsing them as UTF-8 at verification time:
+
+.Parse Headers As UTF-8
+[tabs]
+======
+Java::
++
+[source,java,role="primary"]
+----
+firewall.setAllowedHeaderValues((header) -> {
+    String parsed = new String(header.getBytes(ISO_8859_1), UTF_8);
+    return allowed.matcher(parsed).matches();
+});
+----
+
+Kotlin::
++
+[source,kotlin,role="secondary"]
+----
+firewall.setAllowedHeaderValues {
+    val parsed = String(header.getBytes(ISO_8859_1), UTF_8)
+    return allowed.matcher(parsed).matches()
+}
+----
+======

web/src/main/java/org/springframework/security/web/server/WebFilterChainProxy.java

@@ -22,6 +22,12 @@
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
+import org.springframework.security.web.server.firewall.HttpStatusExchangeRejectedHandler;
+import org.springframework.security.web.server.firewall.ServerExchangeRejectedException;
+import org.springframework.security.web.server.firewall.ServerExchangeRejectedHandler;
+import org.springframework.security.web.server.firewall.ServerWebExchangeFirewall;
+import org.springframework.security.web.server.firewall.StrictServerWebExchangeFirewall;
+import org.springframework.util.Assert;
 import org.springframework.web.server.ServerWebExchange;
 import org.springframework.web.server.WebFilter;
 import org.springframework.web.server.WebFilterChain;
@@ -38,6 +44,10 @@ public class WebFilterChainProxy implements WebFilter {
 
 	private final List<SecurityWebFilterChain> filters;
 
+	private ServerWebExchangeFirewall firewall = new StrictServerWebExchangeFirewall();
+
+	private ServerExchangeRejectedHandler exchangeRejectedHandler = new HttpStatusExchangeRejectedHandler();
+
 	public WebFilterChainProxy(List<SecurityWebFilterChain> filters) {
 		this.filters = filters;
 	}
@@ -48,12 +58,41 @@ public WebFilterChainProxy(SecurityWebFilterChain... filters) {
 
 	@Override
 	public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
+		return this.firewall.getFirewalledExchange(exchange)
+				.flatMap((firewalledExchange) -> filterFirewalledExchange(firewalledExchange, chain))
+				.onErrorResume(ServerExchangeRejectedException.class,
+						(rejected) -> this.exchangeRejectedHandler.handle(exchange, rejected).then(Mono.empty()));
+	}
+
+	private Mono<Void> filterFirewalledExchange(ServerWebExchange firewalledExchange, WebFilterChain chain) {
 		return Flux.fromIterable(this.filters)
-				.filterWhen((securityWebFilterChain) -> securityWebFilterChain.matches(exchange)).next()
-				.switchIfEmpty(chain.filter(exchange).then(Mono.empty()))
+				.filterWhen((securityWebFilterChain) -> securityWebFilterChain.matches(firewalledExchange)).next()
+				.switchIfEmpty(chain.filter(firewalledExchange).then(Mono.empty()))
 				.flatMap((securityWebFilterChain) -> securityWebFilterChain.getWebFilters().collectList())
 				.map((filters) -> new FilteringWebHandler(chain::filter, filters)).map(DefaultWebFilterChain::new)
-				.flatMap((securedChain) -> securedChain.filter(exchange));
+				.flatMap((securedChain) -> securedChain.filter(firewalledExchange));
+	}
+
+	/**
+	 * Protects the application using the provided
+	 * {@link StrictServerWebExchangeFirewall}.
+	 * @param firewall the {@link StrictServerWebExchangeFirewall} to use. Cannot be null.
+	 * @since 5.7.13
+	 */
+	public void setFirewall(ServerWebExchangeFirewall firewall) {
+		Assert.notNull(firewall, "firewall cannot be null");
+		this.firewall = firewall;
+	}
+
+	/**
+	 * Handles {@link ServerExchangeRejectedException} when the
+	 * {@link ServerWebExchangeFirewall} rejects the provided {@link ServerWebExchange}.
+	 * @param exchangeRejectedHandler the {@link ServerExchangeRejectedHandler} to use.
+	 * @since 5.7.13
+	 */
+	public void setExchangeRejectedHandler(ServerExchangeRejectedHandler exchangeRejectedHandler) {
+		Assert.notNull(exchangeRejectedHandler, "exchangeRejectedHandler cannot be null");
+		this.exchangeRejectedHandler = exchangeRejectedHandler;
 	}
 
 }

web/src/main/java/org/springframework/security/web/server/firewall/HttpStatusExchangeRejectedHandler.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2002-2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.security.web.server.firewall;
+
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+import reactor.core.publisher.Mono;
+
+import org.springframework.core.log.LogMessage;
+import org.springframework.http.HttpStatus;
+import org.springframework.web.server.ServerWebExchange;
+
+/**
+ * A simple implementation of {@link ServerExchangeRejectedHandler} that sends an error
+ * with configurable status code.
+ *
+ * @author Rob Winch
+ * @since 5.7.13
+ */
+public class HttpStatusExchangeRejectedHandler implements ServerExchangeRejectedHandler {
+
+	private static final Log logger = LogFactory.getLog(HttpStatusExchangeRejectedHandler.class);
+
+	private final HttpStatus status;
+
+	/**
+	 * Constructs an instance which uses {@code 400} as response code.
+	 */
+	public HttpStatusExchangeRejectedHandler() {
+		this(HttpStatus.BAD_REQUEST);
+	}
+
+	/**
+	 * Constructs an instance which uses a configurable http code as response.
+	 * @param status http status code to use
+	 */
+	public HttpStatusExchangeRejectedHandler(HttpStatus status) {
+		this.status = status;
+	}
+
+	@Override
+	public Mono<Void> handle(ServerWebExchange exchange,
+			ServerExchangeRejectedException serverExchangeRejectedException) {
+		return Mono.fromRunnable(() -> {
+			logger.debug(
+					LogMessage.format("Rejecting request due to: %s", serverExchangeRejectedException.getMessage()),
+					serverExchangeRejectedException);
+			exchange.getResponse().setStatusCode(this.status);
+		});
+	}
+
+}

web/src/main/java/org/springframework/security/web/server/firewall/ServerExchangeRejectedException.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2002-2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.security.web.server.firewall;
+
+/**
+ * Thrown when a {@link org.springframework.web.server.ServerWebExchange} is rejected.
+ *
+ * @author Rob Winch
+ * @since 5.7.13
+ */
+public class ServerExchangeRejectedException extends RuntimeException {
+
+	public ServerExchangeRejectedException(String message) {
+		super(message);
+	}
+
+}