Harden debug bundle filesystem collaborators

internal/core/fileio/fileio.go

@@ -46,6 +46,9 @@ func SafePath(root, name string) (string, error) {
 	if rel == "." || rel == ".." || strings.HasPrefix(rel, ".."+string(filepath.Separator)) {
 		return "", fmt.Errorf("artifact name %q resolves outside file root", name)
 	}
+	if err := rejectSymlinkComponents(absRoot, name); err != nil {
+		return "", err
+	}
 	return target, nil
 }
 
@@ -62,16 +65,44 @@ func WriteBytes(root, name string, data []byte, dirMode, fileMode os.FileMode) e
 	if err != nil {
 		return err
 	}
-	if err := writeAtomic(target, data, dirMode, fileMode); err != nil {
+	if err := os.MkdirAll(filepath.Dir(target), dirMode); err != nil {
+		return fmt.Errorf("write artifact %q: %w", name, err)
+	}
+	if err := rejectSymlinkComponents(root, name); err != nil {
+		return err
+	}
+	if err := writeAtomic(target, data, fileMode); err != nil {
 		return fmt.Errorf("write artifact %q: %w", name, err)
 	}
 	return nil
 }
 
-func writeAtomic(target string, data []byte, dirMode, fileMode os.FileMode) error {
-	if err := os.MkdirAll(filepath.Dir(target), dirMode); err != nil {
-		return err
+func rejectSymlinkComponents(root, name string) error {
+	absRoot, err := filepath.Abs(root)
+	if err != nil {
+		return fmt.Errorf("resolve file root %q: %w", root, err)
 	}
+	current := absRoot
+	for _, component := range strings.Split(filepath.FromSlash(name), string(filepath.Separator)) {
+		if component == "" || component == "." {
+			continue
+		}
+		current = filepath.Join(current, component)
+		info, err := os.Lstat(current)
+		if err != nil {
+			if os.IsNotExist(err) {
+				return nil
+			}
+			return fmt.Errorf("inspect artifact path %q: %w", name, err)
+		}
+		if info.Mode()&os.ModeSymlink != 0 {
+			return fmt.Errorf("artifact path %q must not traverse symbolic links", name)
+		}
+	}
+	return nil
+}
+
+func writeAtomic(target string, data []byte, fileMode os.FileMode) error {
 	temp, err := os.CreateTemp(filepath.Dir(target), "."+filepath.Base(target)+".tmp-*")
 	if err != nil {
 		return err

internal/core/fileio/fileio_test.go

@@ -39,3 +39,18 @@ func TestWriteBytesIsAtomicAndUsesRequestedModes(t *testing.T) {
 		}
 	}
 }
+
+func TestWriteBytesRejectsSymlinkedComponents(t *testing.T) {
+	root := t.TempDir()
+	outside := t.TempDir()
+	if err := os.Symlink(outside, filepath.Join(root, "link")); err != nil {
+		t.Skipf("symbolic links unavailable: %v", err)
+	}
+
+	if err := WriteBytes(root, "link/value", []byte("value"), 0o700, 0o600); err == nil {
+		t.Fatal("WriteBytes accepted a symlinked directory")
+	}
+	if _, err := os.Stat(filepath.Join(outside, "value")); !os.IsNotExist(err) {
+		t.Fatalf("write escaped through symlink: %v", err)
+	}
+}