Reject paths with inner '..' in FileLoader.New to prevent silent misresolution

Author: 0xMH

api/internal/loader/fileloader.go

@@ -147,6 +147,26 @@ func newLoaderAtConfirmedDir(
 	}
 }
 
+// hasInnerDotDot reports whether path contains a ".." after a non-".."
+// component, which causes filepath.Clean to silently absorb preceding
+// segments (e.g. "../../base - ../../shared/prod" becomes "../../shared/prod").
+func hasInnerDotDot(path string) bool {
+	pastPrefix := false
+	for _, part := range strings.Split(filepath.ToSlash(path), "/") {
+		if part == "" || part == "." {
+			continue
+		}
+		if part == ".." {
+			if pastPrefix {
+				return true
+			}
+			continue
+		}
+		pastPrefix = true
+	}
+	return false
+}
+
 // New returns a new Loader, rooted relative to current loader,
 // or rooted in a temp directory holding a git repo clone.
 func (fl *FileLoader) New(path string) (ifc.Loader, error) {
@@ -167,6 +187,13 @@ func (fl *FileLoader) New(path string) (ifc.Loader, error) {
 	if filepath.IsAbs(path) {
 		return nil, fmt.Errorf("new root '%s' cannot be absolute", path)
 	}
+	if hasInnerDotDot(path) {
+		return nil, fmt.Errorf(
+			"path %q is ambiguous: resolves to %q after normalization, "+
+				"which is likely not the intended target; check for YAML "+
+				"indentation errors in your kustomization file",
+			path, filepath.Clean(path))
+	}
 	root, err := filesys.ConfirmDir(fl.fSys, fl.root.Join(path))
 	if err != nil {
 		return nil, errors.WrapPrefixf(err, "%s", ErrRtNotDir.Error())

api/internal/loader/fileloader_test.go

@@ -207,6 +207,60 @@ func TestNewEmptyLoader(t *testing.T) {
 	require.Error(t, err)
 }
 
+func TestLoaderRejectsMalformedPath(t *testing.T) {
+	// A YAML indentation error can collapse two resource entries into one:
+	//   resources:
+	//   - ../../base
+	//    - ../../shared/prod
+	// becomes the single string: "../../base - ../../shared/prod"
+	//
+	// filepath.Clean normalizes this to "../../shared/prod", silently
+	// dropping the "../../base" reference. The loader must reject paths
+	// with inner ".." components that cause this silent absorption.
+	// See https://github.com/kubernetes-sigs/kustomize/issues/5979
+	fSys := filesys.MakeFsInMemory()
+	require.NoError(t, fSys.MkdirAll("/base"))
+	require.NoError(t, fSys.MkdirAll("/shared/prod"))
+	require.NoError(t, fSys.MkdirAll("/overlays/prod1"))
+
+	l1 := NewLoaderOrDie(RestrictionNone, fSys, "/overlays/prod1")
+
+	// The exact bug from issue #5979.
+	_, err := l1.New("../../base - ../../shared/prod")
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "ambiguous")
+
+	// Same structural problem without the YAML artifact.
+	_, err = l1.New("a/b/../../other")
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "ambiguous")
+}
+
+func TestHasInnerDotDot(t *testing.T) {
+	cases := map[string]bool{
+		// Safe: leading ".." only
+		"../base":           false,
+		"../../shared/prod": false,
+		"..":                false,
+		// Safe: no ".." at all
+		"foo/bar":      false,
+		"foo/bar/":     false,
+		"foo//bar":     false,
+		"./foo/bar":    false,
+		"https://root": false,
+		// Dangerous: inner ".." absorbs preceding components
+		"../../base - ../../shared/prod": true,
+		"a/b/../../c":                    true,
+		"foo/../bar":                     true,
+		"a/..":                           true,
+	}
+	for path, want := range cases {
+		t.Run(path, func(t *testing.T) {
+			require.Equal(t, want, hasInnerDotDot(path), "hasInnerDotDot(%q)", path)
+		})
+	}
+}
+
 func TestNewRemoteLoaderDoesNotExist(t *testing.T) {
 	_, err := makeLoader().New("https://example.com/org/repo")
 	require.ErrorContains(t, err, "fetch")