Fix single-file classpath JAR resources being extracted onto the shared temp directory

core/src/main/java/org/testcontainers/utility/MountableFile.java

@@ -250,7 +250,7 @@ private String extractClassPathResourceToTempLocation(final String hostPath) {
                         hostPath,
                         tmpLocation
                     );
-                    copyFromJarToLocation(jarFile, entry, internalPath, tmpLocation);
+                    copyFromJarToLocation(jarFile, entry, tmpLocation);
                 }
             }
         } catch (IOException e) {
@@ -264,7 +264,14 @@ private String extractClassPathResourceToTempLocation(final String hostPath) {
         deleteOnExit(tmpLocation.toPath());
 
         try {
-            return tmpLocation.getCanonicalPath();
+            // Preserve the resource's own path within the JAR underneath the extraction directory
+            // (see #9423). A single extracted file therefore still ends up inside a directory
+            // created specifically for this extraction, rather than being written directly onto
+            // the temp directory's own path - which would leave the extracted file's parent as the
+            // shared system temp directory, breaking any caller that treats the resolved path's
+            // parent as a self-contained context (e.g. building an image from a Dockerfile loaded
+            // via a classpath resource).
+            return new File(tmpLocation, internalPath).getCanonicalPath();
         } catch (IOException e) {
             throw new IllegalStateException(e);
         }
@@ -282,16 +289,18 @@ private File createTempDirectory() {
     }
 
     @SuppressWarnings("ResultOfMethodCallIgnored")
-    private void copyFromJarToLocation(
-        final JarFile jarFile,
-        final JarEntry entry,
-        final String fromRoot,
-        final File toRoot
-    ) throws IOException {
-        String destinationName = entry.getName().replaceFirst(fromRoot, "");
-        File newFile = new File(toRoot, destinationName);
-
-        log.debug("Copying resource {} from JAR file {}", fromRoot, jarFile.getName());
+    private void copyFromJarToLocation(final JarFile jarFile, final JarEntry entry, final File toRoot) throws IOException {
+        File newFile = new File(toRoot, entry.getName());
+
+        // Guard against malformed entries (e.g. containing "../") escaping the extraction directory
+        Path rootPath = toRoot.getCanonicalFile().toPath();
+        if (!newFile.getCanonicalFile().toPath().startsWith(rootPath)) {
+            throw new IOException(
+                "JAR entry " + entry.getName() + " would be extracted outside of the target directory " + toRoot
+            );
+        }
+
+        log.debug("Copying resource {} from JAR file {}", entry.getName(), jarFile.getName());
 
         if (!entry.isDirectory()) {
             // Create parent directories

core/src/test/java/org/testcontainers/utility/MountableFileTest.java

@@ -11,9 +11,16 @@
 import java.io.ByteArrayOutputStream;
 import java.io.File;
 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.LinkedHashMap;
+import java.util.Map;
 import java.util.function.Consumer;
+import java.util.jar.JarEntry;
+import java.util.jar.JarOutputStream;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -53,6 +60,62 @@ void forClasspathResourceFromJarWithAbsolutePath() throws Exception {
         performChecks(mountableFile);
     }
 
+    @Test
+    void forClasspathResourceFileInJarIsExtractedIntoItsOwnDirectory() throws Exception {
+        // see #9423: a single-file classpath resource extracted from a JAR must end up inside a
+        // directory created specifically for this extraction (preserving the resource's own path
+        // within the JAR), rather than being written directly onto the temp *directory*'s own path.
+        // Otherwise the extracted file's parent is the shared system temp directory, which breaks
+        // any caller that treats the resolved path's parent as a self-contained context (e.g.
+        // building an image from a Dockerfile loaded via MountableFile.forClasspathResource(...)).
+        final Map<String, String> entries = new LinkedHashMap<>();
+        entries.put("nested/inside/jar/Dockerfile", "FROM postgres\n");
+        final Path jarFile = createJarWithEntries(entries);
+
+        withJarOnClasspath(
+            jarFile,
+            () -> {
+                final MountableFile mountableFile = MountableFile.forClasspathResource("nested/inside/jar/Dockerfile");
+                final File extractedFile = new File(mountableFile.getFilesystemPath());
+
+                assertThat(extractedFile).as("the resource was extracted to a real file").isFile();
+                assertThat(Files.readString(extractedFile.toPath())).isEqualTo("FROM postgres\n");
+
+                final File parentDir = extractedFile.getParentFile();
+                assertThat(parentDir)
+                    .as("the extracted file's parent is not the shared system temp directory")
+                    .isNotEqualTo(new File(System.getProperty("java.io.tmpdir")));
+                assertThat(parentDir.list())
+                    .as("only the extracted resource lives in its own extraction directory")
+                    .containsExactly("Dockerfile");
+            }
+        );
+    }
+
+    @Test
+    void forClasspathResourceDirectoryInJarPreservesRelativeStructure() throws Exception {
+        // Regression guard: extracting a directory resource from a JAR must still lay out its
+        // files relative to the resolved path exactly as before this fix.
+        final Map<String, String> entries = new LinkedHashMap<>();
+        entries.put("assets/", "");
+        entries.put("assets/dir/", "");
+        entries.put("assets/dir/sub/", "");
+        entries.put("assets/dir/a.txt", "a-content");
+        entries.put("assets/dir/sub/b.txt", "b-content");
+        final Path jarFile = createJarWithEntries(entries);
+
+        withJarOnClasspath(
+            jarFile,
+            () -> {
+                final MountableFile mountableFile = MountableFile.forClasspathResource("assets/dir");
+                final String resolvedPath = mountableFile.getResolvedPath();
+
+                assertThat(Files.readString(new File(resolvedPath, "a.txt").toPath())).isEqualTo("a-content");
+                assertThat(Files.readString(new File(resolvedPath, "sub/b.txt").toPath())).isEqualTo("b-content");
+            }
+        );
+    }
+
     @Test
     void forHostPath() throws Exception {
         final Path file = createTempFile("somepath");
@@ -131,6 +194,38 @@ void noTrailingSlashesInTarEntryNames() throws Exception {
         }
     }
 
+    @NotNull
+    private Path createJarWithEntries(final Map<String, String> entries) throws IOException {
+        final Path jarFile = Files.createTempFile("mountable-file-test", ".jar");
+        jarFile.toFile().deleteOnExit();
+
+        try (JarOutputStream jos = new JarOutputStream(Files.newOutputStream(jarFile))) {
+            for (final Map.Entry<String, String> entry : entries.entrySet()) {
+                jos.putNextEntry(new JarEntry(entry.getKey()));
+                jos.write(entry.getValue().getBytes(StandardCharsets.UTF_8));
+                jos.closeEntry();
+            }
+        }
+
+        return jarFile;
+    }
+
+    private void withJarOnClasspath(final Path jarFile, final ThrowingRunnable runnable) throws Exception {
+        final ClassLoader previousContextClassLoader = Thread.currentThread().getContextClassLoader();
+        try (URLClassLoader jarClassLoader = new URLClassLoader(new URL[] { jarFile.toUri().toURL() }, previousContextClassLoader)) {
+            Thread.currentThread().setContextClassLoader(jarClassLoader);
+            try {
+                runnable.run();
+            } finally {
+                Thread.currentThread().setContextClassLoader(previousContextClassLoader);
+            }
+        }
+    }
+
+    private interface ThrowingRunnable {
+        void run() throws Exception;
+    }
+
     private TarArchiveInputStream intoTarArchive(Consumer<TarArchiveOutputStream> consumer) throws IOException {
         @Cleanup
         final ByteArrayOutputStream baos = new ByteArrayOutputStream();