NiceGUI Reflected XSS in ui.add_css, ui.add_scss, and ui.add_sass via Style Injection

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

6.1 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

EPSS

• CVE-2025-66469: 0.27% estimated exploitation activity in the next 30 days; percentile 19.97 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary A Cross-Site Scripting (XSS) vulnerability exists in ui.addcss, ui.addscss, and ui.addsass functions in NiceGUI (v3.3.1 and earlier). These functions allow developers to inject styles dynamically. However, they lack proper sanitization or encoding for the JavaScript context they generate. An attacker can break out of the intended <style or <script tags by injecting closing tags (e.g., </style or </script), allowing for the execution of arbitrary JavaScript. Details The vulnerability stems from how these functions inject content into the DOM using client.runjavascript (or addheadhtml internally) without sufficient escaping for the transport layer. ui.addcss: Injects content into a <style tag. Input containing </style closes the tag prematurely, allowing subsequent HTML/JS injection. ui.addscss / ui.addsass: These rely on client-side compilation within <script tags. Input containing </script breaks the execution context, allowing XSS. PoC Scenario: A developer allows users to…

Published by the source

8 Dec 2025, 21:30 UTC

Last modified by the source

7 Jul 2026, 17:56 UTC

Source last checked

26 Oct 2026, 13:36 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-72qc-wxch-74mg

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-72qc-wxch-74mg

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# Patch Next advisory brief: CVE-2025-66469

Advisory: NiceGUI Reflected XSS in ui.add_css, ui.add_scss, and ui.add_sass via Style Injection
IDs: CVE-2025-66469, GHSA-72qc-wxch-74mg
Record: https://patchnext.com/advisories/cve-2025-66469 (revision 2, source checked 26 Oct 2026, 13:36 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nicegui 3.4.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nicegui: affected <= 3.3.1; fixed in 3.4.0

## Suggested inspection (read-only)
- `python -m pip show nicegui`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on nicegui.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-72qc-wxch-74mg (OSV record GHSA-72qc-wxch-74mg lists affected ranges: PyPI nicegui >=0 <3.4.0 (fixed))
- Verified: https://github.com/advisories/GHSA-72qc-wxch-74mg (GitHub-reviewed advisory GHSA-72qc-wxch-74mg lists affected ranges: PyPI nicegui >=0 <=3.3.1)

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