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

Severity: medium (CVSS 6.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-01-08 · updated 2026-06-17
Weakness: CWE-79

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.

Affected products

zauberzeug nicegui: from 2.22.0 before 3.5.0 — fixed in 3.5.0

How to fix

Upgrade to a fixed version: zauberzeug nicegui 3.5.0. Check the vendor advisory for mitigations.

References

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

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