diff --git a/spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/config/JsonMessageConverter.java b/spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/config/JsonMessageConverter.java
index 36484a6c9..ac4d30015 100644
--- a/spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/config/JsonMessageConverter.java
+++ b/spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/config/JsonMessageConverter.java
@@ -16,13 +16,12 @@
 
 package org.springframework.cloud.function.context.config;
 
+import java.io.BufferedReader;
+import java.io.InputStreamReader;
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
-import java.net.URI;
 import java.net.URL;
 import java.nio.charset.StandardCharsets;
-import java.nio.file.Files;
-import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.Enumeration;
 import java.util.List;
@@ -68,8 +67,12 @@ public JsonMessageConverter(JsonMapper jsonMapper, MimeType... supportedMimeType
 		try {
 			Enumeration<URL> resources = ClassUtils.getDefaultClassLoader().getResources("META-INF/deserializable.types");
 			while (resources.hasMoreElements()) {
-				URI uri = resources.nextElement().toURI();
-				List<String> lines = Files.readAllLines(Path.of(uri));
+				URL url = resources.nextElement();
+				List<String> lines;
+				try (BufferedReader reader = new BufferedReader(
+						new InputStreamReader(url.openStream(), StandardCharsets.UTF_8))) {
+					lines = reader.lines().toList();
+				}
 				for (String line : lines) {
 					// need to split in case if delimited
 					String[] keys = line.split(",");
diff --git a/spring-cloud-function-context/src/test/java/org/springframework/cloud/function/context/config/JsonMessageConverterAllowlistPackagingTests.java b/spring-cloud-function-context/src/test/java/org/springframework/cloud/function/context/config/JsonMessageConverterAllowlistPackagingTests.java
new file mode 100644
index 000000000..d5540819e
--- /dev/null
+++ b/spring-cloud-function-context/src/test/java/org/springframework/cloud/function/context/config/JsonMessageConverterAllowlistPackagingTests.java
@@ -0,0 +1,130 @@
+/*
+ * Copyright 2026-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.cloud.function.context.config;
+
+import java.io.IOException;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Enumeration;
+import java.util.jar.JarEntry;
+import java.util.jar.JarOutputStream;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+import tools.jackson.databind.ObjectMapper;
+
+import org.springframework.cloud.function.json.JacksonMapper;
+import org.springframework.messaging.Message;
+import org.springframework.messaging.MessageHeaders;
+import org.springframework.messaging.support.MessageBuilder;
+import org.springframework.util.MimeTypeUtils;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class JsonMessageConverterAllowlistPackagingTests {
+
+	private static final String ALLOWLIST = "META-INF/deserializable.types";
+
+	@Test
+	public void readsAllowlistFromDirectoryOnClasspath(@TempDir Path temp) throws Exception {
+		Path directory = temp.resolve("classes");
+		Path metaInf = Files.createDirectories(directory.resolve("META-INF"));
+		Files.writeString(metaInf.resolve("deserializable.types"), Person.class.getName());
+
+		assertThat(convertWithAllowlistAt(directory.toUri().toURL())).isInstanceOf(Person.class);
+	}
+
+	@Test
+	public void readsAllowlistNestedInsideJarOnClasspath(@TempDir Path temp) throws Exception {
+		URL jar = jarContainingAllowlist(temp.resolve("allowlist.jar"), Person.class.getName());
+
+		assertThat(allowlistLoader(jar).getResource(ALLOWLIST))
+				.hasToString("jar:" + jar + "!/META-INF/deserializable.types");
+
+		assertThat(convertWithAllowlistAt(jar))
+				.as("an allowlist nested inside an archive must be read, as it is when an application "
+						+ "is packaged as a Spring Boot fat jar")
+				.isInstanceOf(Person.class);
+	}
+
+	@Test
+	public void ignoresTypeParameterWhenNestedAllowlistDoesNotNameIt(@TempDir Path temp) throws Exception {
+		URL jar = jarContainingAllowlist(temp.resolve("allowlist.jar"), "com.example.SomethingElse");
+
+		assertThat(convertWithAllowlistAt(jar)).isNotInstanceOf(Person.class);
+	}
+
+	private Object convertWithAllowlistAt(URL allowlist) throws IOException {
+		ClassLoader original = Thread.currentThread().getContextClassLoader();
+		try (URLClassLoader loader = allowlistLoader(allowlist)) {
+			Thread.currentThread().setContextClassLoader(loader);
+			JsonMessageConverter converter = new JsonMessageConverter(new JacksonMapper(new ObjectMapper()));
+			Message<String> message = MessageBuilder.withPayload("{\"name\":\"bill\"}")
+					.setHeader(MessageHeaders.CONTENT_TYPE,
+							MimeTypeUtils.APPLICATION_JSON + ";type=" + Person.class.getName())
+					.build();
+			return converter.convertFromInternal(message, Object.class, null);
+		}
+		finally {
+			Thread.currentThread().setContextClassLoader(original);
+		}
+	}
+
+	private URLClassLoader allowlistLoader(URL allowlist) {
+		ClassLoader parent = getClass().getClassLoader();
+		return new URLClassLoader(new URL[] { allowlist }, parent) {
+
+			@Override
+			public URL getResource(String name) {
+				return ALLOWLIST.equals(name) ? findResource(name) : super.getResource(name);
+			}
+
+			@Override
+			public Enumeration<URL> getResources(String name) throws IOException {
+				return ALLOWLIST.equals(name) ? findResources(name) : super.getResources(name);
+			}
+
+		};
+	}
+
+	private URL jarContainingAllowlist(Path jar, String allowlistedType) throws IOException {
+		try (JarOutputStream out = new JarOutputStream(Files.newOutputStream(jar))) {
+			out.putNextEntry(new JarEntry(ALLOWLIST));
+			out.write(allowlistedType.getBytes(StandardCharsets.UTF_8));
+			out.closeEntry();
+		}
+		return jar.toUri().toURL();
+	}
+
+	public static class Person {
+
+		private String name;
+
+		public String getName() {
+			return this.name;
+		}
+
+		public void setName(String name) {
+			this.name = name;
+		}
+
+	}
+
+}