Validate Windows layer paths before conversion

internal/windows/windows.go

@@ -71,11 +71,16 @@ func Windows(layer v1.Layer) (v1.Layer, error) {
 			return nil, fmt.Errorf("reading layer: %w", err)
 		}
 
-		if strings.HasPrefix(header.Name, "Files/") {
+		cleanName, err := cleanLayerPath(header.Name)
+		if err != nil {
+			return nil, err
+		}
+
+		if strings.HasPrefix(cleanName, "Files/") {
 			return nil, fmt.Errorf("file path %q already suitable for Windows", header.Name)
 		}
 
-		header.Name = path.Join("Files", header.Name)
+		header.Name = path.Join("Files", cleanName)
 		header.Format = tar.FormatPAX
 
 		// TODO: this seems to make the file executable on Windows;
@@ -112,3 +117,11 @@ func Windows(layer v1.Layer) (v1.Layer, error) {
 
 	return layer, nil
 }
+
+func cleanLayerPath(name string) (string, error) {
+	cleanName := path.Clean(name)
+	if path.IsAbs(name) || cleanName == ".." || strings.HasPrefix(cleanName, "../") {
+		return "", fmt.Errorf("unsafe file path %q", name)
+	}
+	return cleanName, nil
+}

internal/windows/windows_test.go

@@ -16,12 +16,14 @@ package windows
 
 import (
 	"archive/tar"
+	"bytes"
 	"errors"
 	"io"
 	"reflect"
 	"strings"
 	"testing"
 
+	v1 "github.com/google/go-containerregistry/pkg/v1"
 	"github.com/google/go-containerregistry/pkg/v1/tarball"
 )
 
@@ -79,3 +81,44 @@ func TestWindows(t *testing.T) {
 		t.Errorf("didn't see Files/ directory")
 	}
 }
+
+func TestWindowsRejectsUnsafePaths(t *testing.T) {
+	for _, name := range []string{"../Hives/escaped.txt", "/Hives/escaped.txt"} {
+		t.Run(name, func(t *testing.T) {
+			layer := layerWithFile(t, name)
+			if _, err := Windows(layer); err == nil {
+				t.Fatalf("Windows accepted unsafe path %q", name)
+			}
+		})
+	}
+}
+
+func layerWithFile(t *testing.T, name string) v1.Layer {
+	t.Helper()
+
+	var buf bytes.Buffer
+	tw := tar.NewWriter(&buf)
+	content := []byte("test")
+	if err := tw.WriteHeader(&tar.Header{
+		Name:     name,
+		Typeflag: tar.TypeReg,
+		Mode:     0644,
+		Size:     int64(len(content)),
+	}); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := tw.Write(content); err != nil {
+		t.Fatal(err)
+	}
+	if err := tw.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	layer, err := tarball.LayerFromOpener(func() (io.ReadCloser, error) {
+		return io.NopCloser(bytes.NewReader(buf.Bytes())), nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	return layer
+}