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.
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
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.
| Score | Percentile |
|---|---|
| 0.23% | 45.41% |
| Base score | Version | Severity | Vector |
|---|---|---|---|
| 5.3 | 3.1 | — |
|
| Type | Value |
|---|---|
| GHSA | GHSA-2c2j-9gv5-cj73 ↗ |
| CVE | CVE-2025-54121 ↗ |
| CWE id | Name |
|---|---|
| CWE-770 | Allocation of Resources Without Limits or Throttling |
Vulnerable version ranges and first patched releases as published by GitHub.
| Ecosystem | Package | Vulnerable range | First patched | Vulnerable functions |
|---|---|---|---|---|
| pip | starlette | < 0.47.2 | 0.47.2 | — |