CVE-2026-45554

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.

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.

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.

The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.

CVE-2026-45554 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.12.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-45554? CVE-2026-45554 is a medium-severity allocation of resources without limits or throttling vulnerability in nicegui (pip), affecting versions <= 3.11.1. It is fixed in 3.12.0. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
2. How severe is CVE-2026-45554? CVE-2026-45554 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of nicegui are affected by CVE-2026-45554? nicegui (pip) versions <= 3.11.1 is affected.
4. Is there a fix for CVE-2026-45554? Yes. CVE-2026-45554 is fixed in 3.12.0. Upgrade to this version or later.
5. Is CVE-2026-45554 exploitable, and should I be worried? Whether CVE-2026-45554 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-45554 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-45554? Upgrade nicegui to 3.12.0 or later.