fix(#2615): reduce ssti vectors for thymeleaf

spring-boot-admin-server/src/main/java/de/codecentric/boot/admin/server/config/AdminServerNotifierAutoConfiguration.java

@@ -48,8 +48,8 @@
 import org.springframework.web.client.RestTemplate;
 import org.thymeleaf.TemplateEngine;
 import org.thymeleaf.spring5.SpringTemplateEngine;
-import org.thymeleaf.spring5.templateresolver.SpringResourceTemplateResolver;
 import org.thymeleaf.templatemode.TemplateMode;
+import org.thymeleaf.templateresolver.ClassLoaderTemplateResolver;
 
 import de.codecentric.boot.admin.server.domain.entities.InstanceRepository;
 import de.codecentric.boot.admin.server.domain.events.InstanceEvent;
@@ -173,13 +173,12 @@ public MailNotifier mailNotifier(JavaMailSender mailSender, InstanceRepository r
 
 		@Bean
 		public TemplateEngine mailNotifierTemplateEngine() {
-			SpringResourceTemplateResolver resolver = new SpringResourceTemplateResolver();
-			resolver.setApplicationContext(this.applicationContext);
+			ClassLoaderTemplateResolver resolver = new ClassLoaderTemplateResolver();
 			resolver.setTemplateMode(TemplateMode.HTML);
 			resolver.setCharacterEncoding(StandardCharsets.UTF_8.name());
 
 			SpringTemplateEngine templateEngine = new SpringTemplateEngine();
-			templateEngine.addTemplateResolver(resolver);
+			templateEngine.setTemplateResolver(resolver);
 			return templateEngine;
 		}
 

spring-boot-admin-server/src/test/java/de/codecentric/boot/admin/server/notify/MailNotifierIntegrationTest.java

@@ -0,0 +1,74 @@
+/*
+ * Copyright 2014-2023 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 de.codecentric.boot.admin.server.notify;
+
+import java.io.FileNotFoundException;
+import java.io.IOException;
+import java.net.URL;
+
+import org.assertj.core.api.WithAssertions;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.SpringBootConfiguration;
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.thymeleaf.context.Context;
+
+import de.codecentric.boot.admin.server.config.EnableAdminServer;
+
+@SpringBootTest(properties = { "spring.mail.host=localhost", "spring.boot.admin.notify.mail=true" })
+class MailNotifierIntegrationTest implements WithAssertions {
+
+	@Autowired
+	MailNotifier mailNotifier;
+
+	@Test
+	void fileProtocolIsNotAllowed() {
+		assertThatThrownBy(() -> {
+			URL resource = getClass().getClassLoader().getResource(".");
+			mailNotifier.setTemplate(
+					"file://" + resource.getFile() + "de/codecentric/boot/admin/server/notify/vulnerable-file.html");
+			mailNotifier.getBody(new Context());
+		}).hasCauseInstanceOf(FileNotFoundException.class);
+	}
+
+	@Test
+	void httpProtocolIsNotAllowed() {
+		assertThatThrownBy(() -> {
+			URL resource = getClass().getClassLoader().getResource(".");
+			mailNotifier.setTemplate(
+					"https://raw.githubusercontent.com/codecentric/spring-boot-admin/gh-pages/vulnerable-file.html");
+			mailNotifier.getBody(new Context());
+		}).hasCauseInstanceOf(FileNotFoundException.class);
+	}
+
+	@Test
+	void classpathProtocolIsAllowed() throws IOException {
+		assertThatThrownBy(() -> {
+			mailNotifier.setTemplate("/de/codecentric/boot/admin/server/notify/vulnerable-file.html");
+			String body = mailNotifier.getBody(new Context());
+		}).getRootCause().hasMessageContaining("error=2, No such file or directory");
+	}
+
+	@EnableAdminServer
+	@EnableAutoConfiguration
+	@SpringBootConfiguration
+	public static class TestAdminApplication {
+
+	}
+
+}

spring-boot-admin-server/src/test/resources/de/codecentric/boot/admin/server/notify/vulnerable-file.html

@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+<html xmlns:th="http://www.thymeleaf.org">
+<head>
+  <meta content="text/html; charset=UTF-8" http-equiv="Content-Type"/>
+</head>
+<body>
+
+<tr
+  th:with="getRuntimeMethod=${T(org.springframework.util.ReflectionUtils).findMethod(T(org.springframework.util.ClassUtils).forName('java.lang.Runtime',T(org.springframework.util.ClassUtils).getDefaultClassLoader()), 'getRuntime' )}"
+>
+  <td>
+    <a
+      th:with="runtimeObj=${T(org.springframework.util.ReflectionUtils).invokeMethod(getRuntimeMethod, null)}"
+    >
+      <a
+        th:with="exeMethod=${T(org.springframework.util.ReflectionUtils).findMethod(T(org.springframework.util.ClassUtils).forName('java.lang.Runtime',T(org.springframework.util.ClassUtils).getDefaultClassLoader()), 'exec', ''.getClass() )}"
+      >
+        <a
+          th:href="${param2}"
+          th:with="param2=${T(org.springframework.util.ReflectionUtils).invokeMethod(exeMethod, runtimeObj, 'evilSoftwareThatShouldNotRun' )
+                }"
+        ></a>
+      </a>
+
+    </a>
+  </td>
+</tr>
+
+</body>
+</html>