# CVE-2026-21874: NiceGUI has Redis connection leak via tab storage causes service degradation

> An unauthenticated attacker can exhaust Redis connections by repeatedly opening and closing browser tabs, causing service degradation in NiceGUI applications using Redis-backed storage.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 8 Jan 2026
- **Updated:** 8 Jan 2026
- **Also known as:** GHSA-mp55-g7pj-rvm2, PYSEC-2026-1703

## What is CVE-2026-21874?

CVE-2026-21874 is a medium-severity denial of service vulnerability in zauberzeug nicegui, published on 8 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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.

Weakness: CWE-772 (Missing Release of Resource after Effective Lifetime)

## Which versions are affected by CVE-2026-21874?

3 open source packages ship an affected version, including nicegui. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `nicegui` | PyPI | From 2.10.0 | 3.5.0 |
| `nicegui` | PyPI | From 2.10.0 | 3.5.0 |
| `nicegui` | PyPI | From 2.10.0 | 3.5.0 |

Products: Zauberzeug Nicegui.

## How do I fix CVE-2026-21874?

Upgrade nicegui to 3.5.0 or later.

## Is CVE-2026-21874 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-21874 exists.

## Other vulnerabilities in zauberzeug nicegui

- [CVE-2026-25732](https://insights.sbomify.com/CVE-2026-25732/): NiceGUI's Path Traversal via Unsanitized FileUpload.name Enables Arbitrary File Write (high 7.5)
- [CVE-2025-66645](https://insights.sbomify.com/CVE-2025-66645/): NiceGUI Path Traversal Vulnerability in app.add_media_files() Allows Arbitrary File Reading (high 7.5)
- [CVE-2026-21873](https://insights.sbomify.com/CVE-2026-21873/): Zero-click XSS in all NiceGUI apps which uses `ui.sub_pages` (high 7.2)
- [CVE-2026-33332](https://insights.sbomify.com/CVE-2026-33332/): NiceGUI's unvalidated chunk size parameter in media routes can cause memory exhaustion (medium 6.9)
- [CVE-2026-27156](https://insights.sbomify.com/CVE-2026-27156/): NiceGUI has XSS via Code Injection (medium 6.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-21874 and fix it if it is affected.

What it is: CVE-2026-21874 is a medium-severity denial of service vulnerability in zauberzeug nicegui, published on 8 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-21874 exists.

Affected packages and fixed versions:
- nicegui (PyPI): affected from 2.10.0; fixed in 3.5.0
- nicegui (PyPI): affected from 2.10.0; fixed in 3.5.0
- nicegui (PyPI): affected from 2.10.0; fixed in 3.5.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-21874/ (Markdown: https://insights.sbomify.com/CVE-2026-21874/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21874
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21874
- OSV: https://osv.dev/vulnerability/CVE-2026-21874

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-21874/
Summaries are generated and may contain mistakes; the official description above is the record.