CVE-2026-21871

Summary

XSS risk exists in NiceGUI when developers pass attacker-controlled strings into ui.navigate.history.push() or ui.navigate.history.replace(). These helpers are documented as History API wrappers for updating the browser URL without page reload. However, if the URL argument is embedded into generated JavaScript without proper escaping, a crafted payload can break out of the intended string context and execute arbitrary JavaScript in the victim’s browser.

Applications that do not pass untrusted input into ui.navigate.history.push/replace are not affected.

Details

NiceGUI provides ui.navigate.history.push(url) and ui.navigate.history.replace(url) to update the URL using the browser History API. If an application forwards user-controlled data (e.g., URL path segments, query parameters like next=..., form values, etc.) into these methods, an attacker can inject characters such as quotes and statement terminators to escape the JavaScript string context and execute arbitrary code.

A vulnerable pattern is:

• attacker controls a value (e.g., via the request path),
• the application passes it to ui.navigate.history.push(payload) (or replace).

This is similar in spirit to other NiceGUI XSS advisories:

• ui.html(),ui.chat_message() can cause XSS when developers render untrusted input as HTML (XSS risk/footgun).
• ui.interactive_image had XSS via unsanitized SVG content and was handled as a security advisory with a fix and severity rating.

Because ui.navigate.history.* is expected to accept a URL (data) rather than executable code, the library should escape/encode the argument before emitting JavaScript.

PoC

Create a simple app

from nicegui import ui

@ui.page('/')
def index():
    # A link/button a victim could click (attacker can also send the URL directly)
    ui.button('open crafted path', on_click=lambda: ui.navigate.to('/%22);alert(document.domain);//'))

@ui.page('/{payload:path}')
def victim(payload: str):
    ui.label(f'payload = {payload!r}')

    # Vulnerable use: forwarding attacker-controlled path to history.push
    ui.button('trigger', on_click=lambda: ui.navigate.history.push(payload))

ui.run()