CVE-2025-54121

When parsing a multi-part form with large files (greater than the default max spool size) starlette will block the main thread to roll the file over to disk. This blocks the event thread which means we can't accept new connections.

Details

Please see this discussion for details: https://github.com/encode/starlette/discussions/2927#discussioncomment-13721403. In summary the following UploadFile code (copied from here) has a minor bug. Instead of just checking for self._in_memory we should also check if the additional bytes will cause a rollover.

    @property
    def _in_memory(self) -> bool:
        # check for SpooledTemporaryFile._rolled
        rolled_to_disk = getattr(self.file, "_rolled", True)
        return not rolled_to_disk

    async def write(self, data: bytes) -> None:
        if self.size is not None:
            self.size += len(data)

        if self._in_memory:
            self.file.write(data)
        else:
            await run_in_threadpool(self.file.write, data)

I have already created a PR which fixes the problem: https://github.com/encode/starlette/pull/2962

PoC

See the discussion here for steps on how to reproduce.

To be honest, very low and not many users will be impacted. Parsing large forms is already CPU intensive so the additional IO block doesn't slow down starlette that much on systems with modern HDDs/SSDs. If someone is running on tape they might see a greater impact.

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-2025-54121 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 (0.47.2); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2025-54121? CVE-2025-54121 is a medium-severity allocation of resources without limits or throttling vulnerability in starlette (pip), affecting versions < 0.47.2. It is fixed in 0.47.2. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
2. How severe is CVE-2025-54121? CVE-2025-54121 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 starlette are affected by CVE-2025-54121? starlette (pip) versions < 0.47.2 is affected.
4. Is there a fix for CVE-2025-54121? Yes. CVE-2025-54121 is fixed in 0.47.2. Upgrade to this version or later.
5. Is CVE-2025-54121 exploitable, and should I be worried? Whether CVE-2025-54121 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-2025-54121 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-2025-54121? Upgrade starlette to 0.47.2 or later.