GHSA-mhpg-c27v-6mxr
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,handleStateEventis executed.
https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.js#L38-L39
handleStateEventemitssubpagesopenevent.
https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.js#L22-L25
SubPagesRouter(used byui.sub_pages), lisnening onsubpagesopen,handleopenruns.
https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/subpagesrouter.py#L18-L22
handleopenfinds anySubPagesand runs_show()on them
https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/subpagesrouter.py#L63-L71
- If the if-logic is followed or debug prints are added, it can be found that it calls
self.handlescrolling(match, behavior='smooth')directly
https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.py#L76-L100
- CULPRIT
handlescrollingrunsscrollto_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.
<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
- 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.
Package Versions Affected
Automatically patch vulnerabilities without upgrading
CVSS Version
Related Resources
References
https://github.com/zauberzeug/nicegui/security/advisories/GHSA-mhpg-c27v-6mxr, https://nvd.nist.gov/vuln/detail/CVE-2026-21873, https://github.com/zauberzeug/nicegui, https://github.com/zauberzeug/nicegui/releases/tag/v3.5.0
Basic Information
