CVE-2026-21871

Description

### 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 ```python 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() ``` #### Run the app ```bash python app.py ``` #### Trigger 1. Open `http://localhost:8080/` 2. Click `open crafted path` 3. Click `trigger` Expected result: JavaScript executes (an alert showing `document.domain`). ### Impact - Vulnerability type: DOM-based XSS - Attack vector: attacker-controlled input embedded into JavaScript via `ui.navigate.history.push/replace` - Affected users: any NiceGUI-based application that forwards untrusted input into `ui.navigate.history.push()` or `ui.navigate.history.replace()` - Potential outcomes: client-side code execution, phishing UI injection, and other typical XSS impacts