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:
- On
pushstate, handleStateEvent is executed.
https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.js#L38-L39
handleStateEvent emits sub_pages_open event.
https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.js#L22-L25
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
_handle_open finds any SubPages and runs _show() on them
https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/sub_pages_router.py#L63-L71
- 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
- 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
- Not use
ui.sub_pages
- 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 responseAppendix
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.
References
evnchn Reporter
falkoschindler Remediation reviewer
Summary
An unsafe implementation in the
pushstateevent listener used byui.sub_pagesallows 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:
pushstate,handleStateEventis executed.https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.js#L38-L39
handleStateEventemitssub_pages_openevent.https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.js#L22-L25
SubPagesRouter(used byui.sub_pages), lisnening onsub_pages_open,_handle_openruns.https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/sub_pages_router.py#L18-L22
_handle_openfinds anySubPagesand runs_show()on themhttps://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/sub_pages_router.py#L63-L71
self._handle_scrolling(match, behavior='smooth')directlyhttps://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.py#L76-L100
_handle_scrollingruns_scroll_to_fragmentas there is a fragment, which runs vulnerable JS if thefragment(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.
Impact
Any page which uses
ui.sub_pagesand 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
appvia FastAPI methods, and thatui.sub_pagesis actively promoted as the new modern way to create Single-Page Applications (SPA).Patch
ui.sub_pagesAppendix
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.
References