CVE-2025-66469

A Cross-Site Scripting (XSS) vulnerability exists in ui.add_css, ui.add_scss, and ui.add_sass 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.run_javascript (or add_head_html internally) without sufficient escaping for the transport layer.

• ui.add_css: Injects content into a <style> tag. Input containing </style> closes the tag prematurely, allowing subsequent HTML/JS injection.
• ui.add_scss / ui.add_sass: 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 customize a theme color via a URL parameter.

from nicegui import ui

@ui.page('/')
def main(color: str = 'blue'):
    # Vulnerable implementation of dynamic theming
    ui.add_css(f'.q-btn {{ background-color: {color} !important; }}')
    ui.button('Click Me')

ui.run(port=8082)

Attack Vector:
Accessing the following URL executes arbitrary JavaScript:
http://localhost:8082/?color=red;}</style><img src=x onerror=alert(document.domain)><style>

• Type: Reflected XSS
• Severity: Moderate
• Affected Components: Applications using ui.add_css, ui.add_scss, or ui.add_sass with untrusted input (e.g., dynamic theming based on user input).

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2025-66469 has a CVSS score of 6.1 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.4.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2025-66469? CVE-2025-66469 is a medium-severity cross-site scripting (XSS) vulnerability in nicegui (pip), affecting versions <= 3.3.1. It is fixed in 3.4.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2025-66469? CVE-2025-66469 has a CVSS score of 6.1 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of nicegui are affected by CVE-2025-66469? nicegui (pip) versions <= 3.3.1 is affected.
4. Is there a fix for CVE-2025-66469? Yes. CVE-2025-66469 is fixed in 3.4.0. Upgrade to this version or later.
5. Is CVE-2025-66469 exploitable, and should I be worried? Whether CVE-2025-66469 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-66469 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-66469? Upgrade nicegui to 3.4.0 or later.