diff --git a/serve.py b/serve.py
index e45c56c1..5c7ec7e9 100644
--- a/serve.py
+++ b/serve.py
@@ -34,15 +34,26 @@ def translate_path(self, path):
             # HACK: double-slashes are being generated by JS
             path = path.replace('//', '/')
             rel_path = path[len('/analysis/docs/'):]
-            return os.path.join(DOCS_SITE, rel_path)
+            return self._join_under(DOCS_SITE, rel_path)
         # Serve /analysis-book/* from BOOK_SITE
         elif path.startswith('/analysis/'):
             rel_path = path[len('/analysis/'):]
-            return os.path.join(BOOK_SITE, rel_path)
+            return self._join_under(BOOK_SITE, rel_path)
         # Otherwise, serve nothing (could return a non-existent path)
         else:
             raise FileNotFoundError(f"File not found: {path}")
 
+    @staticmethod
+    def _join_under(root, rel_path):
+        """Join rel_path under root, rejecting path traversal via .. components."""
+        candidate = os.path.abspath(os.path.join(root, rel_path))
+        try:
+            if os.path.commonpath([root, candidate]) != root:
+                return os.path.join(root, '.path-rejected')
+        except ValueError:
+            return os.path.join(root, '.path-rejected')
+        return candidate
+
 
 if __name__ == '__main__':
     import argparse