NiceGUI is a Python-based UI framework. Versions 2.24.2 and below are at risk for Cross-Site Scripting (XSS) when developers render unescaped user input into the DOM using ui.html(). NiceGUI did not enforce HTML or JavaScript sanitization, so applications that directly combine components like ui.input() with ui.html() or ui.chat_message with HTML content without escaping may allow attackers to execute arbitrary JavaScript in the user’s browser. Applications that do not pass untrusted input into ui.html() are not affected. This issue is fixed in version 3.0.0.

Affected: PyPI: nicegui

### Summary

A Cross-Site Scripting (XSS) risk exists in NiceGUI when developers render unescaped user input into the DOM using `ui.html()`. Before version 3.0, NiceGUI does not enforce HTML or JavaScript sanitization, so applications that directly combine components like `ui.input()` with `ui.html()` without escaping may allow attackers to execute arbitrary JavaScript in the user’s browser. Same holds for `ui.chat_message` with HTML content.

Applications that directly reflect user input via `ui.html()` (or `ui.chat_message` in HTML mode) are affected. This may lead to client-side code execution (e.g., session hijacking or phishing). Applications that do not pass untrusted input into ui.html() are not affected.

### Details

NiceGUI allows developers to bind user input directly into the DOM using `ui.html()` or `ui.chat_message()`. However, the library does not enforce any HTML or JavaScript sanitization, which potentially creates a dangerous attack surface for developers unaware of this behavior.

The vulnerable code path appears when combining these:

```python ui.input("XSS Input:", on_change=inject) def inject(e): ui.html(f'{e.value}') ```

In this setup, any input provided by the user is rendered **verbatim** into the page’s DOM via innerHTML, enabling injection of script-based payloads.

### PoC (Proof of Concept)

1. Create a simple app:

```python from nicegui import ui

@ui.page('/') def main(): def inject(e): ui.html(f'{e.value}')…

Affected: PyPI: nicegui

### Summary

A Cross-Site Scripting (XSS) risk exists in NiceGUI when developers render unescaped user input into the DOM using `ui.html()`. Before version 3.0, NiceGUI does not enforce HTML or JavaScript sanitization, so applications that directly combine components like `ui.input()` with `ui.html()` without escaping may allow attackers to execute arbitrary JavaScript in the user’s browser. Same holds for `ui.chat_message` with HTML content.

Applications that directly reflect user input via `ui.html()` (or `ui.chat_message` in HTML mode) are affected. This may lead to client-side code execution (e.g., session hijacking or phishing). Applications that do not pass untrusted input into ui.html() are not affected.

### Details

NiceGUI allows developers to bind user input directly into the DOM using `ui.html()` or `ui.chat_message()`. However, the library does not enforce any HTML or JavaScript sanitization, which potentially creates a dangerous attack surface for developers unaware of this behavior.

The vulnerable code path appears when combining these:

```python ui.input("XSS Input:", on_change=inject) def inject(e): ui.html(f'{e.value}') ```

In this setup, any input provided by the user is rendered **verbatim** into the page’s DOM via innerHTML, enabling injection of script-based payloads.

### PoC (Proof of Concept)

1. Create a simple app:

```python from nicegui import ui

@ui.page('/') def main(): def inject(e): ui.html(f'{e.value}')…