diff --git a/backend/onyx/chat/process_message.py b/backend/onyx/chat/process_message.py
index a26d20d70a..40e978c3ef 100644
--- a/backend/onyx/chat/process_message.py
+++ b/backend/onyx/chat/process_message.py
@@ -830,6 +830,7 @@ def stream_chat_message_objects(
             db_session=db_session,
             persona=persona,
             actual_user_input=message_text,
+            user_id=user_id,
         )
         if not search_tool_override_kwargs_for_user_files:
             latest_query_files.extend(in_memory_user_files)
diff --git a/backend/onyx/chat/user_files/parse_user_files.py b/backend/onyx/chat/user_files/parse_user_files.py
index ee3f93e7c7..37ef93345a 100644
--- a/backend/onyx/chat/user_files/parse_user_files.py
+++ b/backend/onyx/chat/user_files/parse_user_files.py
@@ -1,9 +1,11 @@
+from uuid import UUID
+
 from sqlalchemy.orm import Session
 
 from onyx.db.models import Persona
 from onyx.db.models import UserFile
 from onyx.file_store.models import InMemoryChatFile
-from onyx.file_store.utils import get_user_files
+from onyx.file_store.utils import get_user_files_as_user
 from onyx.file_store.utils import load_in_memory_chat_files
 from onyx.tools.models import SearchToolOverrideKwargs
 from onyx.utils.logger import setup_logger
@@ -18,6 +20,8 @@ def parse_user_files(
     db_session: Session,
     persona: Persona,
     actual_user_input: str,
+    # should only be None if auth is disabled
+    user_id: UUID | None,
 ) -> tuple[list[InMemoryChatFile], list[UserFile], SearchToolOverrideKwargs | None]:
     """
     Parse user files and folders into in-memory chat files and create search tool override kwargs.
@@ -29,6 +33,7 @@ def parse_user_files(
         db_session: Database session
         persona: Persona to calculate available tokens
         actual_user_input: User's input message for token calculation
+        user_id: User ID to validate file ownership
 
     Returns:
         Tuple of (
@@ -49,9 +54,10 @@ def parse_user_files(
         db_session,
     )
 
-    user_file_models = get_user_files(
+    user_file_models = get_user_files_as_user(
         user_file_ids or [],
         user_folder_ids or [],
+        user_id,
         db_session,
     )
 
diff --git a/backend/onyx/context/search/pipeline.py b/backend/onyx/context/search/pipeline.py
index 433e16d7dd..877ab12c16 100644
--- a/backend/onyx/context/search/pipeline.py
+++ b/backend/onyx/context/search/pipeline.py
@@ -42,6 +42,8 @@ from onyx.utils.threadpool_concurrency import FunctionCall
 from onyx.utils.threadpool_concurrency import run_functions_in_parallel
 from onyx.utils.timing import log_function_time
 from onyx.utils.variable_functionality import fetch_ee_implementation_or_noop
+from shared_configs.configs import MULTI_TENANT
+from shared_configs.contextvars import get_current_tenant_id
 
 logger = setup_logger()
 
@@ -180,7 +182,11 @@ class SearchPipeline:
         filters = IndexFilters(
             user_file_ids=user_file_ids or [],
             user_folder_ids=user_folder_ids or [],
+            # NOTE: this can be None, since it's assumed that the user_file_ids / user_folder_ids
+            # have already been verified as owned by the user running this query
+            # TODO: make this more robust
             access_control_list=None,
+            tenant_id=get_current_tenant_id() if MULTI_TENANT else None,
         )
 
         # Use a simplified query that skips all unnecessary processing
diff --git a/backend/onyx/file_store/utils.py b/backend/onyx/file_store/utils.py
index 0fc3d7fb70..9f3a56da6c 100644
--- a/backend/onyx/file_store/utils.py
+++ b/backend/onyx/file_store/utils.py
@@ -2,6 +2,7 @@ import base64
 from collections.abc import Callable
 from io import BytesIO
 from typing import cast
+from uuid import UUID
 from uuid import uuid4
 
 import requests
@@ -245,6 +246,26 @@ def get_user_files(
     return user_files
 
 
+def get_user_files_as_user(
+    user_file_ids: list[int],
+    user_folder_ids: list[int],
+    user_id: UUID | None,
+    db_session: Session,
+) -> list[UserFile]:
+    """
+    Fetches all UserFile database records for a given user.
+    """
+    user_files = get_user_files(user_file_ids, user_folder_ids, db_session)
+    for user_file in user_files:
+        # Note: if user_id is None, then all files should be None as well
+        # (since auth must be disabled in this case)
+        if user_file.user_id != user_id:
+            raise ValueError(
+                f"User {user_id} does not have access to file {user_file.id}"
+            )
+    return user_files
+
+
 def save_file_from_url(url: str) -> str:
     """NOTE: using multiple sessions here, since this is often called
     using multithreading. In practice, sharing a session has resulted in