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._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.
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.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | starlette | < 0.47.2 | 0.47.2 |
References
- https://github.com/encode/starlette/security/advisories/GHSA-2c2j-9gv5-cj73
- https://nvd.nist.gov/vuln/detail/CVE-2025-54121
- https://github.com/encode/starlette/commit/9f7ec2eb512fcc3fe90b43cb9dd9e1d08696bec1
- https://github.com/encode/starlette
- https://github.com/encode/starlette/blob/fa5355442753f794965ae1af0f87f9fec1b9a3de/starlette/datastructures.py#L436C5-L447C14
- https://github.com/encode/starlette/discussions/2927#discussioncomment-13721403
- https://pypi.org/project/starlette
- https://github.com/advisories/GHSA-2c2j-9gv5-cj73
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L- Weakness (CWE)
CWE-770- Platform
- Python (PyPI)
- Package
starlette- Disclosed
- 21.07.2025
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-1941 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-2c2j-9gv5-cj73 - CC BY 4.0
Vulnerability history: starlette
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 10
- First → latest
- 02.2023 06.2026
- Without a fix
- 0
- Exploited in attacks
- 1
Severity breakdown
- High 5
- Medium 4
- Low 1
Disclosures per year
- 2023: 2
- 2024: 1
- 2025: 2
- 2026: 5
-
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