diff --git a/app/routes/document_routes.py b/app/routes/document_routes.py
index 15f5b8d0..0d207b1c 100644
--- a/app/routes/document_routes.py
+++ b/app/routes/document_routes.py
@@ -1066,6 +1066,7 @@ async def embed_file_upload(
 
 @router.post("/query_multiple")
 async def query_embeddings_by_file_ids(request: Request, body: QueryMultipleBody):
+    user_authorized = get_user_id(request)
     try:
         # Get the embedding of the query text
         embedding = get_cached_query_embedding(body.query)
@@ -1085,13 +1086,30 @@ async def query_embeddings_by_file_ids(request: Request, body: QueryMultipleBody
 
         documents = _apply_distance_threshold(documents)
 
+        # Authorization: return only chunks the requester owns (or unowned ones),
+        # mirroring the per-document check in /query. The filter above scopes by
+        # file_id only, so without this any caller could read another tenant's
+        # documents by supplying their file_id.
+        authorized_documents = [
+            (doc, score)
+            for doc, score in documents
+            if doc.metadata.get("user_id") in (None, user_authorized)
+        ]
+        withheld = len(documents) - len(authorized_documents)
+        if withheld:
+            logger.warning(
+                "query_multiple: withheld %d chunk(s) not owned by user %s",
+                withheld,
+                user_authorized,
+            )
+
         # Ensure documents list is not empty
-        if not documents:
+        if not authorized_documents:
             raise HTTPException(
                 status_code=404, detail="No documents found for the given query"
             )
 
-        return documents
+        return authorized_documents
     except HTTPException as http_exc:
         logger.error(
             "HTTP Exception in query_embeddings_by_file_ids | Status: %d | Detail: %s",
diff --git a/tests/test_main.py b/tests/test_main.py
index a7fce89a..baa2becf 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -279,3 +279,43 @@ def test_extract_text_from_file(tmp_path, auth_headers):
     assert json_data["file_id"] == "test_text_123"
     assert json_data["filename"] == "test_text_extraction.txt"
     assert json_data["known_type"] is True  # text files are known types
+
+
+def test_query_multiple_filters_unauthorized(auth_headers, monkeypatch):
+    """/query_multiple must withhold chunks owned by another user.
+
+    Regression: it filtered by file_id only (no user_id check), so any caller
+    could read another tenant's documents by supplying their file_id.
+    """
+    from app.services.vector_store.async_pg_vector import AsyncPgVector
+
+    async def cross_tenant_results(self, embedding, k, filter=None, executor=None):
+        return [
+            (
+                Document(
+                    page_content="mine",
+                    metadata={"file_id": "f1", "user_id": "testuser"},
+                ),
+                0.1,
+            ),
+            (
+                Document(
+                    page_content="theirs",
+                    metadata={"file_id": "f2", "user_id": "someoneelse"},
+                ),
+                0.2,
+            ),
+        ]
+
+    monkeypatch.setattr(
+        AsyncPgVector,
+        "asimilarity_search_with_score_by_vector",
+        cross_tenant_results,
+    )
+
+    data = {"query": "q", "file_ids": ["f1", "f2"], "k": 4}
+    response = client.post("/query_multiple", json=data, headers=auth_headers)
+    assert response.status_code == 200, response.text
+    contents = [row[0]["page_content"] for row in response.json()]
+    assert "mine" in contents
+    assert "theirs" not in contents