CVE-2025-54121: Starlette has possible denial-of-service vector when parsing large files in multipart forms

Summary 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.inmemory we should also check if the additional bytes will cause a rollover.

python

@property def inmemory(self) -> bool: # check for SpooledTemporaryFile.rolled rolledtodisk = getattr(self.file, "rolled", True) return not rolledtodisk

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

if self.inmemory: self.file.write(data) else: await runinthreadpool(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.

Impact 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.