Reject backslashes in SpringTemplateLoader template names

Closes gh-37054

spring-context-support/src/main/java/org/springframework/ui/freemarker/SpringTemplateLoader.java

@@ -72,6 +72,9 @@ public class SpringTemplateLoader implements TemplateLoader {
 		if (logger.isDebugEnabled()) {
 			logger.debug("Looking for FreeMarker template with name [" + name + "]");
 		}
+		if (name.indexOf('\\') != -1) {
+			return null;
+		}
 		Resource resource = this.resourceLoader.getResource(this.templateLoaderPath + name);
 		return (resource.exists() ? resource : null);
 	}

spring-context-support/src/test/java/org/springframework/ui/freemarker/SpringTemplateLoaderTests.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2002-present 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.ui.freemarker;
+
+import java.io.File;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import org.springframework.core.io.DefaultResourceLoader;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link SpringTemplateLoader}.
+ *
+ * @author Sébastien Deleuze
+ */
+class SpringTemplateLoaderTests {
+
+	@Test
+	void findTemplateSourceResolvesTemplateInsidePath(@TempDir Path tempDir) throws Exception {
+		Path templates = Files.createDirectory(tempDir.resolve("templates"));
+		Files.writeString(templates.resolve("hello.ftl"), "Hello");
+		SpringTemplateLoader loader = new SpringTemplateLoader(new DefaultResourceLoader(),
+				"file:" + templates.toAbsolutePath() + File.separator);
+		assertThat(loader.findTemplateSource("hello.ftl")).isNotNull();
+	}
+
+	@Test
+	void findTemplateSourceRejectsBackslash(@TempDir Path tempDir) throws Exception {
+		Path templates = Files.createDirectory(tempDir.resolve("templates"));
+		Files.writeString(tempDir.resolve("other.txt"), "other");
+		SpringTemplateLoader loader = new SpringTemplateLoader(new DefaultResourceLoader(),
+				"file:" + templates.toAbsolutePath() + File.separator);
+		assertThat(loader.findTemplateSource("..\\other.txt")).isNull();
+	}
+
+}