v4: harden image download symlink writes

2 files changed, 81 insertions, 0 deletions

diff --git a/internal/image/download.go b/internal/image/download.go
index d66b946..6689a9c 100644
--- a/internal/image/download.go
+++ b/internal/image/download.go
@@ -61,6 +61,9 @@ func (d *Downloader) DownloadImage(ctx context.Context, result *SearchResult, ou
 			return fmt.Errorf("create output dir %q: %w", dir, err)
 		}
 	}
+	if err := ensureNoSymlinkAncestors(outputPath); err != nil {
+		return fmt.Errorf("validate output path %q: %w", outputPath, err)
+	}
 
 	if d.options != nil && !d.options.OverwriteExisting {
 		if _, err := os.Stat(outputPath); err == nil {
@@ -229,6 +232,50 @@ func joinWithinBaseDir(baseDir, name string) (string, error) {
 	return fullPath, nil
 }
 
+func ensureNoSymlinkAncestors(path string) error {
+	absPath, err := filepath.Abs(path)
+	if err != nil {
+		return fmt.Errorf("resolve output path: %w", err)
+	}
+
+	absPath = filepath.Clean(absPath)
+	dir := filepath.Dir(absPath)
+	root := filepath.VolumeName(absPath) + string(os.PathSeparator)
+	if filepath.VolumeName(absPath) == "" {
+		root = string(os.PathSeparator)
+	}
+
+	for {
+		if dir == root || dir == "." || dir == string(os.PathSeparator) {
+			break
+		}
+
+		if info, err := os.Lstat(dir); err == nil {
+			if info.Mode()&os.ModeSymlink != 0 {
+				return fmt.Errorf("path component %q is a symlink", dir)
+			}
+		} else if !os.IsNotExist(err) {
+			return fmt.Errorf("stat path component %q: %w", dir, err)
+		}
+
+		next := filepath.Dir(dir)
+		if next == dir {
+			break
+		}
+		dir = next
+	}
+
+	if info, err := os.Lstat(absPath); err == nil {
+		if info.Mode()&os.ModeSymlink != 0 {
+			return fmt.Errorf("output path %q is a symlink", absPath)
+		}
+	} else if !os.IsNotExist(err) {
+		return fmt.Errorf("stat output path %q: %w", absPath, err)
+	}
+
+	return nil
+}
+
 func sanitizeFileName(name string) string {
 	replacer := strings.NewReplacer(
 		"/", "_",
diff --git a/internal/image/download_test.go b/internal/image/download_test.go
index b9a6e31..84e8f5f 100644
--- a/internal/image/download_test.go
+++ b/internal/image/download_test.go
@@ -189,3 +189,37 @@ func TestDownloadImageRejectsNilResult(t *testing.T) {
 		t.Fatal("expected error for nil result")
 	}
 }
+
+func TestDownloadImageRejectsSymlinkAncestor(t *testing.T) {
+	t.Parallel()
+
+	baseDir := t.TempDir()
+	outsideDir := t.TempDir()
+	linkDir := filepath.Join(baseDir, "link")
+	if err := os.Symlink(outsideDir, linkDir); err != nil {
+		t.Fatalf("Symlink() error = %v", err)
+	}
+
+	provider := &mockDownloaderProvider{payload: "image-bytes"}
+	d := NewDownloader(provider, &DownloadOptions{
+		OutputDir:         baseDir,
+		CreateDir:         true,
+		OverwriteExisting: true,
+	})
+
+	outputPath := filepath.Join(linkDir, "escape.png")
+	err := d.DownloadImage(context.Background(), &SearchResult{
+		URL:         "https://example.com/image.png",
+		Source:      Gemini,
+		Attribution: "safe attribution",
+	}, outputPath)
+	if err == nil {
+		t.Fatal("expected error for symlink ancestor")
+	}
+	if !strings.Contains(err.Error(), "symlink") {
+		t.Fatalf("DownloadImage() error = %v, want symlink rejection", err)
+	}
+	if _, statErr := os.Stat(filepath.Join(outsideDir, "escape.png")); !os.IsNotExist(statErr) {
+		t.Fatalf("unexpected file write outside output dir: %v", statErr)
+	}
+}