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PYSEC-2026-1702

HighCVSS 7.2 / 10
Published Jul 7, 2026·Last modified Jul 7, 2026
Affected Components(1)
PyPI logonicegui
2.22.0 – 3.5.0
Description

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>
<img width="1429" height="643" alt="image" src="https://github.com/user-attachments/assets/310dbb5c-65d5-44f2-8417-dcf044829bc6" />

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.
Risk Scores
Base Score
7.2

The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The vulnerability can affect other systems as well, not just the initial system. There is a low impact on the confidentiality of the information. There is a low impact on the integrity of the data.

Threat Intelligence
6.6

Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.

EPSS
0.23%

The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.

Exploit
Not available

We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.

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