Sanitize inputs in default reactive HTML error view

This commit uses HTML escaping to sanitize error inputs that are
displayed in the default reactive HTML error view.

Fixes gh-11582

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/reactive/error/AbstractErrorWebExceptionHandler.java

 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -38,6 +38,7 @@ import org.springframework.web.reactive.function.server.ServerRequest;
 import org.springframework.web.reactive.function.server.ServerResponse;
 import org.springframework.web.reactive.result.view.ViewResolver;
 import org.springframework.web.server.ServerWebExchange;
+import org.springframework.web.util.HtmlUtils;
 
 /**
  * Abstract base class for {@link ErrorWebExceptionHandler} implementations.
@@ -191,9 +192,11 @@ public abstract class AbstractErrorWebExceptionHandler
 				.append("<p>This application has no configured error view, so you are seeing this as a fallback.</p>")
 				.append("<div id='created'>").append(timestamp.toString())
 				.append("</div>").append("<div>There was an unexpected error (type=")
-				.append(error.get("error")).append(", status=")
-				.append(error.get("status")).append(").</div>").append("<div>")
-				.append(error.get("message")).append("</div>").append("</body></html>");
+				.append(HtmlUtils.htmlEscape(error.get("error").toString())).append(", status=")
+				.append(HtmlUtils.htmlEscape(error.get("status").toString()))
+				.append(").</div>").append("<div>")
+				.append(HtmlUtils.htmlEscape(error.get("message").toString()))
+				.append("</div>").append("</body></html>");
 		return responseBody.syncBody(builder.toString());
 	}
 

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/reactive/error/DefaultErrorWebExceptionHandlerIntegrationTests.java

@@ -220,6 +220,27 @@ public class DefaultErrorWebExceptionHandlerIntegrationTests {
 				});
 	}
 
+	@Test
+	public void escapeHtmlInDefaultErrorView() throws Exception {
+		this.contextRunner
+				.withPropertyValues("spring.mustache.prefix=classpath:/unknown/")
+				.run((context) -> {
+					WebTestClient client = WebTestClient.bindToApplicationContext(context)
+							.build();
+					String body = client.get().uri("/html").accept(MediaType.TEXT_HTML)
+							.exchange().expectStatus()
+							.isEqualTo(HttpStatus.INTERNAL_SERVER_ERROR).expectHeader()
+							.contentType(MediaType.TEXT_HTML).expectBody(String.class)
+							.returnResult().getResponseBody();
+					assertThat(body).contains("Whitelabel Error Page")
+							.doesNotContain("<script>")
+							.contains("&lt;script&gt;");
+					this.output.expect(
+							allOf(containsString("Failed to handle request [GET /html]"),
+									containsString("IllegalStateException")));
+				});
+	}
+
 	@Test
 	public void responseCommitted() throws Exception {
 		this.contextRunner.run((context) -> {
@@ -253,6 +274,11 @@ public class DefaultErrorWebExceptionHandlerIntegrationTests {
 						Mono.error(new IllegalStateException("already committed!")));
 			}
 
+			@GetMapping("/html")
+			public String htmlEscape() {
+				throw new IllegalStateException("<script>");
+			}
+
 			@PostMapping(path = "/bind", produces = "application/json")
 			@ResponseBody
 			public String bodyValidation(@Valid @RequestBody DummyBody body) {