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PYSEC-2026-2680
No affected components available
Summary
Two FastAPI routes that serve per-component static assets in NiceGUI accept a sub-path parameter that may resolve to a directory rather than a file. Requests that resolve to a directory raise an unhandled RuntimeError inside Starlette's FileResponse, which Uvicorn writes to the server log as a full traceback. Because the routes are reachable without authentication, a remote attacker can amplify log volume and consume disk and log-pipeline capacity on any publicly reachable NiceGUI server. There is no impact to confidentiality or integrity.
Details
The affected routes are the per-component resource route (added in v1.4.6) and the ESM module route (added in v3.0.0). Both join a user-supplied path segment with a registered base directory and pass the result to FileResponse. The existing existence check uses pathlib.Path.exists(), which returns True for directories — so a request whose sub-path resolves to a directory passes the guard and triggers an unhandled exception inside Starlette.
FastAPI has no default handler for RuntimeError, so each such request results in a 500 response and a multi-frame traceback in the server log.
Other NiceGUI-served paths (/static/..., /components/..., /libraries/...) are not affected; they do not use the same sub-path-to-FileResponse pattern.
Impact
A remote, unauthenticated attacker can repeatedly trigger the error condition with crafted requests. Each request emits roughly 100 lines of traceback in a default setup, and more when additional middleware layers are present. At sustained request rates this can:
- exhaust disk space on hosts with default log retention,
- saturate downstream log-shipping pipelines,
- generate alert fatigue or mask other events in monitoring.
There is no remote code execution, no path traversal, and no data exposure beyond the absolute installation path that already appears in any uncaught exception trace.
Workarounds
For deployments that cannot upgrade immediately:
- Place NiceGUI behind a reverse proxy that rejects requests where the path after
/_nicegui/<version>/esm/<key>/or/_nicegui/<version>/resources/<key>/is empty. - Rate-limit the
/_nicegui/prefix at the proxy. - Configure log rotation aggressively for the affected service.
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 impact is confined to the system where the vulnerability exists. There is a low impact on the availability of the system.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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