fix(R-1187 b): restored links point only downward — no absolute target, no '..', no link on the way

Third automated security review finding, red-proved.

controller/internal/appexport/r1187_abdfg_test.go

@@ -248,3 +248,29 @@ func TestR1187b_NoLinkThroughALinkedParent(t *testing.T) {
 		t.Fatalf("the refused link must be listed, got %v", refused)
 	}
 }
+
+// (b), the security review's third case: a link TARGET that passes through a link already in the folder („evil" → an
+// outside folder, then „x" → „evil/y") reads as inside by its text and really points outside. Restored links may only
+// point DOWNWARD to real folders: no absolute target, no „..", no link on the way. RED-PROOF: the text check created „x".
+func TestR1187b_NoLinkTargetThroughALink(t *testing.T) {
+	outside := t.TempDir()
+	dest := t.TempDir()
+	os.Symlink(outside, filepath.Join(dest, "evil"))
+	tarPath := filepath.Join(t.TempDir(), "d.tar")
+	f, _ := os.Create(tarPath)
+	tw := tar.NewWriter(f)
+	tw.WriteHeader(&tar.Header{Name: "x", Typeflag: tar.TypeSymlink, Linkname: "evil/y"})
+	tw.WriteHeader(&tar.Header{Name: "up", Typeflag: tar.TypeSymlink, Linkname: "sub/../sub"})
+	tw.Close()
+	f.Close()
+	old := lchownFn
+	lchownFn = func(*os.Root, string, int, int) error { return nil }
+	defer func() { lchownFn = old }()
+	refused, _ := extractTar(tarPath, dest)
+	if _, err := os.Lstat(filepath.Join(dest, "x")); err == nil {
+		t.Fatal("a link whose target passes through an outside-pointing link was created")
+	}
+	if strings.Join(refused, ",") != "x,up" {
+		t.Fatalf("both links must be refused and listed, got %v", refused)
+	}
+}

controller/internal/appexport/restore.go

@@ -1203,6 +1203,34 @@ func extractTarGz(tgzPath, destDir string) error {
 	}
 }
 
+// safeDownwardLink is the rule for a restored link: a relative target with no „.." component, and no symbolic link on
+// the way — not among the folders of the link's own path, not among the existing components of its target.
+func safeDownwardLink(rt *os.Root, name, target string) bool {
+	if target == "" || filepath.IsAbs(target) {
+		return false
+	}
+	for _, part := range strings.Split(filepath.ToSlash(target), "/") {
+		if part == ".." {
+			return false
+		}
+	}
+	if parentThroughLink(rt, name) {
+		return false
+	}
+	acc := filepath.Dir(name)
+	for _, part := range strings.Split(filepath.Clean(target), string(os.PathSeparator)) {
+		acc = filepath.Join(acc, part)
+		fi, err := rt.Lstat(acc)
+		if err != nil {
+			break // not there (yet): nothing further down can be a link that exists now
+		}
+		if fi.Mode()&os.ModeSymlink != 0 {
+			return false
+		}
+	}
+	return true
+}
+
 // parentThroughLink is true when any folder on name's path (inside the root) is a symbolic link.
 func parentThroughLink(rt *os.Root, name string) bool {
 	dir := filepath.Dir(name)
@@ -1285,23 +1313,16 @@ func extractTar(tarPath, destDir string) ([]string, error) {
 			out.Close()
 			rt.Chmod(name, os.FileMode(hdr.Mode).Perm())
 		case tar.TypeSymlink:
+			// A restored link may only point DOWNWARD inside the folder, through real folders: no absolute target, no
+			// „..", and no link on the way — neither on the link's own path nor in its target. Such a link can never
+			// resolve outside the app's folder, whatever else the folder holds. Everything else is refused and listed.
+			// (Three rounds of the automated security review: a link written through, a linked parent, a target through
+			// a link. Pinned by TestR1187b_*.)
+			if !safeDownwardLink(rt, name, hdr.Linkname) {
+				refused = append(refused, hdr.Name)
+				continue
+			}
 			dst := hdr.Linkname
-			if filepath.IsAbs(dst) {
-				refused = append(refused, hdr.Name)
-				continue
-			}
-			// The text check below is exact only when no folder on the link's path is itself a link (a „a" → „." makes
-			// „a/b/l" really „b/l"): such an entry is refused (the security review's second case, pinned by
-			// TestR1187b_NoLinkThroughALinkedParent).
-			if parentThroughLink(rt, name) {
-				refused = append(refused, hdr.Name)
-				continue
-			}
-			resolved := filepath.Clean(filepath.Join(filepath.Dir(filepath.Join(root, name)), dst))
-			if resolved != root && !strings.HasPrefix(resolved, root+string(os.PathSeparator)) {
-				refused = append(refused, hdr.Name)
-				continue
-			}
 			if dir := filepath.Dir(name); dir != "." {
 				if err := rt.MkdirAll(dir, 0755); err != nil {
 					return refused, err