diff --git a/api/apps/restful_apis/document_api.py b/api/apps/restful_apis/document_api.py
index f57fe6b8ae..a4d68c2e00 100644
--- a/api/apps/restful_apis/document_api.py
+++ b/api/apps/restful_apis/document_api.py
@@ -23,7 +23,7 @@ from quart import request, make_response
 from peewee import OperationalError
 from pydantic import ValidationError
 
-from api.apps import login_required
+from api.apps import current_user, login_required
 from api.constants import FILE_NAME_LEN_LIMIT, IMG_BASE64_PREFIX
 from api.apps.services.document_api_service import validate_document_update_fields, map_doc_keys, \
     map_doc_keys_with_run_status, update_document_name_only, update_chunk_method, update_document_status_only, \
@@ -1859,7 +1859,16 @@ async def batch_update_document_status(tenant_id, dataset_id):
 @manager.route("/documents/<doc_id>/preview", methods=["GET"])  # noqa: F821
 @login_required
 async def get(doc_id):
+    """Return the raw file bytes for a document the requesting user is authorized to read.
+
+    The user must belong to the tenant that owns the document's knowledge base; otherwise
+    the response is indistinguishable from a missing document to avoid cross-tenant ID
+    enumeration.
+    """
     try:
+        if not DocumentService.accessible(doc_id, current_user.id):
+            return get_data_error_result(message="Document not found!")
+
         e, doc = DocumentService.get_by_id(doc_id)
         if not e:
             return get_data_error_result(message="Document not found!")
@@ -1884,10 +1893,19 @@ async def get(doc_id):
 @login_required
 @add_tenant_id_to_kwargs
 async def download_attachment(tenant_id=None, doc_id=None, attachment_id=None):
+    """Stream a document's underlying file to the requesting user.
+
+    Mirrors the authorization model of the preview endpoint: the user must belong
+    to the tenant that owns the document's knowledge base. A denial returns the
+    same "Document not found!" response so the endpoint cannot be used to
+    enumerate doc ids across tenants.
+    """
     try:
         # Keep backward compatibility with older callers and unit tests that still
         # pass `attachment_id` instead of the route parameter name.
         doc_id = doc_id or attachment_id
+        if not DocumentService.accessible(doc_id, current_user.id):
+            return get_data_error_result(message="Document not found!")
         ext = request.args.get("ext", "markdown")
         data = await thread_pool_exec(settings.STORAGE_IMPL.get, tenant_id, doc_id)
         response = await make_response(data)
diff --git a/test/testcases/test_web_api/test_document_app/test_document_metadata.py b/test/testcases/test_web_api/test_document_app/test_document_metadata.py
index 5a843cdc3a..71bf32d565 100644
--- a/test/testcases/test_web_api/test_document_app/test_document_metadata.py
+++ b/test/testcases/test_web_api/test_document_app/test_document_metadata.py
@@ -343,6 +343,31 @@ class TestDocumentMetadataUnit:
 
     def test_get_route_not_found_success_and_exception_unit(self, document_app_module, monkeypatch):
         module = document_app_module
+
+        # Cross-tenant access is denied -> "Document not found!" (no ID enumeration).
+        # Stub get_by_id to a valid document so the test can only pass via the
+        # accessible() early return; if that check ever regresses, the route would
+        # proceed and the assertions below would no longer match.
+        accessible_calls = []
+
+        def fake_accessible_denied(doc_id, user_id):
+            accessible_calls.append((doc_id, user_id))
+            return False
+
+        monkeypatch.setattr(module.DocumentService, "accessible", fake_accessible_denied)
+        monkeypatch.setattr(
+            module.DocumentService,
+            "get_by_id",
+            lambda _doc_id: (True, SimpleNamespace(name="real.bin", type=module.FileType.OTHER.value)),
+        )
+        res = _run(module.get("doc1"))
+        assert res["code"] == RetCode.DATA_ERROR
+        assert "Document not found!" in res["message"]
+        assert accessible_calls == [("doc1", "user-1")]
+
+        # From here on the user is authorized; exercise the original branches.
+        monkeypatch.setattr(module.DocumentService, "accessible", lambda _doc_id, _user_id: True)
+
         monkeypatch.setattr(module.DocumentService, "get_by_id", lambda _doc_id: (False, None))
         res = _run(module.get("doc1"))
         assert res["code"] == RetCode.DATA_ERROR
@@ -380,6 +405,22 @@ class TestDocumentMetadataUnit:
         module = document_app_module
         monkeypatch.setattr(module, "request", _DummyRequest(args={"ext": "abc"}))
 
+        # Cross-tenant access is denied -> "Document not found!" (no ID enumeration).
+        accessible_calls = []
+
+        def fake_accessible_denied(doc_id, user_id):
+            accessible_calls.append((doc_id, user_id))
+            return False
+
+        monkeypatch.setattr(module.DocumentService, "accessible", fake_accessible_denied)
+        res = _run(module.download_attachment(attachment_id="att1"))
+        assert res["code"] == RetCode.DATA_ERROR
+        assert "Document not found!" in res["message"]
+        assert accessible_calls == [("att1", "user-1")]
+
+        # From here on the user is authorized; exercise the original branches.
+        monkeypatch.setattr(module.DocumentService, "accessible", lambda _doc_id, _user_id: True)
+
         async def fake_thread_pool_exec(*_args, **_kwargs):
             return b"attachment"