# CVE-2026-21873: Zero-click XSS in all NiceGUI apps which uses `ui.sub_pages`

> Attackers can execute arbitrary JavaScript in NiceGUI applications via cross-site iframe manipulation of URL fragments.

- **Severity:** High 7.2 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N`)
- **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-mhpg-c27v-6mxr, PYSEC-2026-1702

## What is CVE-2026-21873?

CVE-2026-21873 is a high-severity cross-site scripting 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 2.22.0 to 3.4.1, an unsafe implementation in the pushstate event listener used by ui.sub_pages allows an attacker to manipulate the fragment identifier of the URL, which they can do despite being cross-site, using an iframe. This issue has been patched in version 3.5.0.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-21873?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `nicegui` | PyPI | From 2.22.0 | 3.5.0 |
| `nicegui` | PyPI | From 2.22.0 | 3.5.0 |
| `nicegui` | PyPI | From 2.22.0 | 3.5.0 |

Products: Zauberzeug Nicegui.

## How do I fix CVE-2026-21873?

Upgrade nicegui to 3.5.0 or later.

## Is CVE-2026-21873 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-21873 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-2026-33332](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)
- [CVE-2026-25516](https://insights.sbomify.com/CVE-2026-25516/): NiceGUI's XSS vulnerability in ui.markdown() allows arbitrary JavaScript execution through unsanitized HTML content (medium 6.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-21873 and fix it if it is affected.

What it is: CVE-2026-21873 is a high-severity cross-site scripting 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-2026-21873 exists.

Affected packages and fixed versions:
- nicegui (PyPI): affected from 2.22.0; fixed in 3.5.0
- nicegui (PyPI): affected from 2.22.0; fixed in 3.5.0
- nicegui (PyPI): affected from 2.22.0; 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-2026-21873/ (Markdown: https://insights.sbomify.com/CVE-2026-21873/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21873
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21873
- OSV: https://osv.dev/vulnerability/CVE-2026-21873

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-21873/
Summaries are generated and may contain mistakes; the official description above is the record.