diff --git a/stone/backend.py b/stone/backend.py
index 5d34160d..bf2fdf6d 100644
--- a/stone/backend.py
+++ b/stone/backend.py
@@ -178,6 +178,10 @@ def _record_output_path(self, output_path):
         self.output_manifest.add_output(self.target_folder_path, output_path)
         return True
 
+    def _validate_output_path(self, output_path):
+        # type: (str) -> None
+        _relative_output_path(self.target_folder_path, output_path)
+
     @contextmanager
     def output_to_relative_path(self, relative_path, mode='wb'):
         # type: (typing.Text, typing.Text) -> typing.Iterator[None]
@@ -188,6 +192,7 @@ def output_to_relative_path(self, relative_path, mode='wb'):
         Clears the output buffer on enter and exit.
         """
         full_path = os.path.join(self.target_folder_path, relative_path)
+        self._validate_output_path(full_path)
         if self._record_output_path(full_path):
             self.clear_output_buffer()
             yield
@@ -208,6 +213,7 @@ def output_to_relative_path(self, relative_path, mode='wb'):
 
     def copy_to_path(self, src, dst, *copy_args, **copy_kwargs):
         output_path = os.path.join(dst, os.path.basename(src)) if os.path.isdir(dst) else dst
+        self._validate_output_path(output_path)
         if self._record_output_path(output_path):
             return output_path
         return shutil.copy(src, dst, *copy_args, **copy_kwargs)
diff --git a/stone/backends/swift.py b/stone/backends/swift.py
index 4588ddba..79130e10 100644
--- a/stone/backends/swift.py
+++ b/stone/backends/swift.py
@@ -270,6 +270,7 @@ def _write_output_in_target_folder(self, output, file_name):
         if not os.path.exists(full_path):
             os.mkdir(full_path)
         full_path = os.path.join(full_path, file_name)
+        self._validate_output_path(full_path)
         if self._record_output_path(full_path):
             return
         with open(full_path, "w", encoding='utf-8') as fh:
diff --git a/test/test_output_manifest.py b/test/test_output_manifest.py
index 7b444588..54426e47 100644
--- a/test/test_output_manifest.py
+++ b/test/test_output_manifest.py
@@ -74,5 +74,53 @@ def generate(self, api):
         self.assertEqual(existing_files, [])
 
 
+class TestOutputRootValidation(unittest.TestCase):
+
+    def test_output_to_relative_path_rejects_parent_paths(self):
+        class ValidatingBackend(CodeBackend):
+            preserve_aliases = True
+
+            def generate(self, api):
+                pass
+
+        with tempfile.TemporaryDirectory() as output_root:
+            backend = ValidatingBackend(output_root, [])
+
+            with self.assertRaises(AssertionError):
+                with backend.output_to_relative_path('../Generated.py'):
+                    backend.emit('generated = True')
+
+    def test_copy_to_path_rejects_parent_paths(self):
+        class ValidatingBackend(Backend):
+            preserve_aliases = True
+
+            def generate(self, api):
+                pass
+
+        with tempfile.NamedTemporaryFile() as source_file:
+            with tempfile.TemporaryDirectory() as output_root:
+                backend = ValidatingBackend(output_root, [])
+
+                with self.assertRaises(AssertionError):
+                    backend.copy_to_path(
+                        source_file.name,
+                        os.path.join(output_root, '..', 'Copied.py'))
+
+    def test_swift_output_rejects_parent_paths(self):
+        class ValidatingBackend(SwiftBaseBackend):
+            preserve_aliases = True
+
+            def generate(self, api):
+                pass
+
+        with tempfile.TemporaryDirectory() as output_root:
+            backend = ValidatingBackend(output_root, [])
+
+            with self.assertRaises(AssertionError):
+                backend._write_output_in_target_folder(
+                    'final class Generated {}',
+                    '../Generated.swift')
+
+
 if __name__ == '__main__':
     unittest.main()