diff --git a/backend/api/cloud/operations.py b/backend/api/cloud/operations.py
index 9b33d5a..7048cb9 100644
--- a/backend/api/cloud/operations.py
+++ b/backend/api/cloud/operations.py
@@ -28,6 +28,7 @@ Admin users are blocked by get_regular_user (T-13-01).
 """
 from __future__ import annotations
 
+import urllib.parse
 import uuid
 from typing import Optional
 
@@ -362,11 +363,13 @@ async def download_cloud_file(
             detail="Provider temporarily unavailable — please try again",
         )
 
-    safe_filename = filename.replace('"', "'")
+    # RFC 6266 percent-encoding — prevents header injection via special characters
+    # (newlines, semicolons, etc.) that str.replace('"', "'") does not cover.
+    encoded_filename = urllib.parse.quote(filename, safe=" !#$&'()*+,-./:;<=>?@[]^_`{|}~")
     return Response(
         content=file_bytes,
         media_type=content_type,
-        headers={"Content-Disposition": f'attachment; filename="{safe_filename}"'},
+        headers={"Content-Disposition": f"attachment; filename*=UTF-8''{encoded_filename}"},
     )