diff --git a/backend/open_webui/routers/files.py b/backend/open_webui/routers/files.py
index 68ef37afe4..6e3271d7ec 100644
--- a/backend/open_webui/routers/files.py
+++ b/backend/open_webui/routers/files.py
@@ -57,64 +57,7 @@ log = logging.getLogger(__name__)
 router = APIRouter()
 
 
-############################
-# Check if the current user has access to a file through any knowledge bases the user may be in.
-############################
-
-
-# TODO: Optimize this function to use the knowledge_file table for faster lookups.
-def has_access_to_file(
-    file_id: Optional[str],
-    access_type: str,
-    user=Depends(get_verified_user),
-    db: Optional[Session] = None,
-) -> bool:
-    file = Files.get_file_by_id(file_id, db=db)
-    log.debug(f"Checking if user has {access_type} access to file")
-    if not file:
-        raise HTTPException(
-            status_code=status.HTTP_404_NOT_FOUND,
-            detail=ERROR_MESSAGES.NOT_FOUND,
-        )
-
-    # Check if the file is associated with any knowledge bases the user has access to
-    knowledge_bases = Knowledges.get_knowledges_by_file_id(file_id, db=db)
-    user_group_ids = {
-        group.id for group in Groups.get_groups_by_member_id(user.id, db=db)
-    }
-    for knowledge_base in knowledge_bases:
-        if knowledge_base.user_id == user.id or AccessGrants.has_access(
-            user_id=user.id,
-            resource_type="knowledge",
-            resource_id=knowledge_base.id,
-            permission=access_type,
-            user_group_ids=user_group_ids,
-            db=db,
-        ):
-            return True
-
-    knowledge_base_id = file.meta.get("collection_name") if file.meta else None
-    if knowledge_base_id:
-        knowledge_bases = Knowledges.get_knowledge_bases_by_user_id(
-            user.id, access_type, db=db
-        )
-        for knowledge_base in knowledge_bases:
-            if knowledge_base.id == knowledge_base_id:
-                return True
-
-    # Check if the file is associated with any channels the user has access to
-    channels = Channels.get_channels_by_file_id_and_user_id(file_id, user.id, db=db)
-    if access_type == "read" and channels:
-        return True
-
-    # Check if the file is associated with any chats the user has access to
-    # TODO: Granular access control for chats
-    chats = Chats.get_shared_chats_by_file_id(file_id, db=db)
-    if chats:
-        return True
-
-    return False
-
+from open_webui.utils.access_control.files import has_access_to_file
 
 ############################
 # Upload File
diff --git a/backend/open_webui/routers/retrieval.py b/backend/open_webui/routers/retrieval.py
index 4c27ffea47..5292546285 100644
--- a/backend/open_webui/routers/retrieval.py
+++ b/backend/open_webui/routers/retrieval.py
@@ -37,6 +37,7 @@ from langchain_text_splitters import (
 from langchain_core.documents import Document
 
 from open_webui.models.files import FileModel, FileUpdateForm, Files
+from open_webui.utils.access_control.files import has_access_to_file
 from open_webui.models.knowledge import Knowledges
 from open_webui.storage.provider import Storage
 from open_webui.internal.db import get_session, get_db
@@ -2579,6 +2580,30 @@ async def query_collection_handler(
     form_data: QueryCollectionsForm,
     user=Depends(get_verified_user),
 ):
+    # Ownership validation: prevent accessing other users' private collections
+    if user.role != "admin":
+        for collection_name in form_data.collection_names:
+            if collection_name.startswith("user-memory-"):
+                owner_id = collection_name[len("user-memory-") :]
+                if owner_id != user.id:
+                    raise HTTPException(
+                        status_code=status.HTTP_403_FORBIDDEN,
+                        detail=ERROR_MESSAGES.ACCESS_PROHIBITED,
+                    )
+            elif collection_name.startswith("file-"):
+                file_id = collection_name[len("file-") :]
+                file = Files.get_file_by_id(file_id)
+                if file and file.user_id != user.id:
+                    if not has_access_to_file(
+                        file_id=file_id,
+                        access_type="read",
+                        user=user,
+                    ):
+                        raise HTTPException(
+                            status_code=status.HTTP_403_FORBIDDEN,
+                            detail=ERROR_MESSAGES.ACCESS_PROHIBITED,
+                        )
+
     try:
         if request.app.state.config.ENABLE_RAG_HYBRID_SEARCH and (
             form_data.hybrid is None or form_data.hybrid
diff --git a/backend/open_webui/tools/builtin.py b/backend/open_webui/tools/builtin.py
index 330baf1318..a59df3813f 100644
--- a/backend/open_webui/tools/builtin.py
+++ b/backend/open_webui/tools/builtin.py
@@ -1627,7 +1627,7 @@ async def view_file(
 
     try:
         from open_webui.models.files import Files
-        from open_webui.routers.files import has_access_to_file
+        from open_webui.utils.access_control.files import has_access_to_file
 
         user_id = __user__.get("id")
         user_role = __user__.get("role", "user")
diff --git a/backend/open_webui/utils/access_control.py b/backend/open_webui/utils/access_control/__init__.py
similarity index 100%
rename from backend/open_webui/utils/access_control.py
rename to backend/open_webui/utils/access_control/__init__.py
diff --git a/backend/open_webui/utils/access_control/files.py b/backend/open_webui/utils/access_control/files.py
new file mode 100644
index 0000000000..b863c404f3
--- /dev/null
+++ b/backend/open_webui/utils/access_control/files.py
@@ -0,0 +1,71 @@
+import logging
+from typing import Optional, Any
+
+from open_webui.models.users import UserModel
+from open_webui.models.files import Files
+from open_webui.models.knowledge import Knowledges
+from open_webui.models.channels import Channels
+from open_webui.models.chats import Chats
+from open_webui.models.groups import Groups
+from open_webui.models.access_grants import AccessGrants
+
+log = logging.getLogger(__name__)
+
+
+def has_access_to_file(
+    file_id: Optional[str],
+    access_type: str,
+    user: UserModel,
+    db: Optional[Any] = None,
+) -> bool:
+    """
+    Check if a user has the specified access to a file through any of:
+    - Knowledge bases (ownership or access grants)
+    - Channels the user is a member of
+    - Shared chats
+
+    NOTE: This does NOT check direct file ownership — callers should check
+    file.user_id == user.id separately before calling this.
+    """
+    file = Files.get_file_by_id(file_id, db=db)
+    log.debug(f"Checking if user has {access_type} access to file")
+    if not file:
+        return False
+
+    # Check if the file is associated with any knowledge bases the user has access to
+    knowledge_bases = Knowledges.get_knowledges_by_file_id(file_id, db=db)
+    user_group_ids = {
+        group.id for group in Groups.get_groups_by_member_id(user.id, db=db)
+    }
+    for knowledge_base in knowledge_bases:
+        if knowledge_base.user_id == user.id or AccessGrants.has_access(
+            user_id=user.id,
+            resource_type="knowledge",
+            resource_id=knowledge_base.id,
+            permission=access_type,
+            user_group_ids=user_group_ids,
+            db=db,
+        ):
+            return True
+
+    knowledge_base_id = file.meta.get("collection_name") if file.meta else None
+    if knowledge_base_id:
+        knowledge_bases = Knowledges.get_knowledge_bases_by_user_id(
+            user.id, access_type, db=db
+        )
+        for knowledge_base in knowledge_bases:
+            if knowledge_base.id == knowledge_base_id:
+                return True
+
+    # Check if the file is associated with any channels the user has access to
+    channels = Channels.get_channels_by_file_id_and_user_id(file_id, user.id, db=db)
+    if access_type == "read" and channels:
+        return True
+
+    # Check if the file is associated with any chats the user has access to
+    # TODO: Granular access control for chats
+    chats = Chats.get_shared_chats_by_file_id(file_id, db=db)
+    if chats:
+        return True
+
+    return False