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PYSEC-2026-1697

MediumCVSS 6.1 / 10
Published Jul 7, 2026·Last modified Jul 7, 2026
Affected Components(0)

No affected components available

Description

Summary

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>

Impact

  • 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).
Risk Scores
Base Score
6.1

The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. The attacker needs the user to perform some action, like clicking a link. The vulnerability can affect other systems as well, not just the initial system. There is a low impact on the confidentiality of the information. There is a low impact on the integrity of the data.

Threat Intelligence
5.6

Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.

EPSS
0.23%

The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.

Exploit
Not available

We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.

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Medium severity
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