diff --git a/backend/open_webui/routers/notes.py b/backend/open_webui/routers/notes.py
index 1308be67ae..128b13add3 100644
--- a/backend/open_webui/routers/notes.py
+++ b/backend/open_webui/routers/notes.py
@@ -30,6 +30,7 @@ from open_webui.utils.access_control import (
     has_public_read_access_grant,
     has_public_write_access_grant,
 )
+from open_webui.utils.access_control.files import has_access_to_note_files
 from open_webui.utils.auth import get_admin_user, get_verified_user
 from pydantic import BaseModel
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -533,6 +534,13 @@ async def update_note_by_id(
     ):
         raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=ERROR_MESSAGES.DEFAULT())
 
+    if (
+        form_data.data
+        and 'files' in form_data.data
+        and not await has_access_to_note_files(note, form_data.data['files'], user, db=db)
+    ):
+        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=ERROR_MESSAGES.DEFAULT())
+
     if form_data.access_grants is not None:
         form_data.access_grants = await filter_allowed_access_grants(
             await Config.get('user.permissions'),
diff --git a/backend/open_webui/socket/main.py b/backend/open_webui/socket/main.py
index d4a5d3f647..86bfd10142 100644
--- a/backend/open_webui/socket/main.py
+++ b/backend/open_webui/socket/main.py
@@ -54,6 +54,7 @@ from open_webui.tasks import (
     stop_item_tasks,
 )
 from open_webui.utils.access_control import has_permission
+from open_webui.utils.access_control.files import has_access_to_note_files
 from open_webui.utils.auth import get_verified_user_by_token
 from open_webui.utils.chat_id import is_saved_chat_id
 from open_webui.utils.json_codec import SOCKETIO_JSON, JSONCodec, dumps_bytes
@@ -1097,6 +1098,13 @@ async def yjs_document_update(sid, data):
                 log.warning(f'User {user.get("id")} does not have write access to note {note_id}. Rejecting update.')
                 return
 
+            snapshot = data.get('data')
+            if isinstance(snapshot, dict) and 'files' in snapshot:
+                editor = await Users.get_user_by_id(user['id'])
+                if not editor or not await has_access_to_note_files(note, snapshot['files'], editor):
+                    await sio.emit('error', {'message': 'Cannot attach files you cannot access'}, room=sid)
+                    return
+
         update = data.get('update')  # List of bytes from frontend
 
         # The document lock makes initialization first-write-wins across workers.
diff --git a/backend/open_webui/utils/access_control/files.py b/backend/open_webui/utils/access_control/files.py
index a30e482215..c8a10910da 100644
--- a/backend/open_webui/utils/access_control/files.py
+++ b/backend/open_webui/utils/access_control/files.py
@@ -8,7 +8,7 @@ from open_webui.models.folders import FolderModel
 from open_webui.models.groups import Groups
 from open_webui.models.knowledge import Knowledges
 from open_webui.models.models import Models
-from open_webui.models.notes import Notes
+from open_webui.models.notes import NoteModel, Notes
 from open_webui.models.users import UserModel, Users
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -17,6 +17,35 @@ log = logging.getLogger(__name__)
 FOLDER_FILE_TYPES = {'file', 'collection', 'note'}
 
 
+def get_note_file_ids(entries: list[dict] | None) -> set[str]:
+    if not isinstance(entries, list):
+        return set()
+    return {
+        entry['id']
+        for entry in entries
+        if isinstance(entry, dict) and entry.get('type') == 'file' and isinstance(entry.get('id'), str) and entry['id']
+    }
+
+
+async def has_access_to_note_files(
+    note: NoteModel, entries: list[dict] | None, user: UserModel, db: AsyncSession | None = None
+) -> bool:
+    """Check the editor's read access to newly attached files."""
+    if entries is not None and not isinstance(entries, list):
+        return False
+
+    # Existing attachments may belong to another editor; only additions need permission.
+    added_ids = get_note_file_ids(entries) - get_note_file_ids((note.data or {}).get('files'))
+    if not added_ids or user.role == 'admin':
+        return True
+
+    group_ids = {group.id for group in await Groups.get_groups_by_member_id(user.id, db=db, include_inherited=True)}
+    for file_id in added_ids:
+        if not await has_access_to_file(file_id, 'read', user, db=db, user_group_ids=group_ids):
+            return False
+    return True
+
+
 async def has_access_to_file(
     file_id: str | None,
     access_type: str,