id: PYSEC-2026-1941
published: "2026-07-07T16:02:58.086064Z"
modified: "2026-07-07T17:25:25.776263Z"
aliases:
- CVE-2025-54121
- GHSA-2c2j-9gv5-cj73
summary: Starlette has possible denial-of-service vector when parsing large files in multipart forms
details: "### Summary\nWhen parsing a multi-part form with large files (greater than the [default max spool size](https://github.com/encode/starlette/blob/fa5355442753f794965ae1af0f87f9fec1b9a3de/starlette/formparsers.py#L126)) `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.\n\n### Details\nPlease see this discussion for details: https://github.com/encode/starlette/discussions/2927#discussioncomment-13721403. In summary the following UploadFile code (copied from [here](https://github.com/encode/starlette/blob/fa5355442753f794965ae1af0f87f9fec1b9a3de/starlette/datastructures.py#L436C5-L447C14)) has a minor bug. Instead of just checking for `self._in_memory` we should also check if the additional bytes will cause a rollover.\n\n```python\n\n    @property\n    def _in_memory(self) -> bool:\n        # check for SpooledTemporaryFile._rolled\n        rolled_to_disk = getattr(self.file, \"_rolled\", True)\n        return not rolled_to_disk\n\n    async def write(self, data: bytes) -> None:\n        if self.size is not None:\n            self.size += len(data)\n\n        if self._in_memory:\n            self.file.write(data)\n        else:\n            await run_in_threadpool(self.file.write, data)\n```\n\nI have already created a PR which fixes the problem: https://github.com/encode/starlette/pull/2962\n\n\n### PoC\nSee the discussion [here](https://github.com/encode/starlette/discussions/2927#discussioncomment-13721403) for steps on how to reproduce.\n\n### Impact\nTo 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:
- package:
    name: starlette
    ecosystem: PyPI
    purl: pkg:pypi/starlette
  ranges:
  - type: ECOSYSTEM
    events:
    - introduced: "0"
    - fixed: 0.47.2
  versions:
  - 0.1.0
  - 0.1.1
  - 0.1.10
  - 0.1.11
  - 0.1.12
  - 0.1.13
  - 0.1.14
  - 0.1.15
  - 0.1.16
  - 0.1.17
  - 0.1.2
  - 0.1.3
  - 0.1.4
  - 0.1.5
  - 0.1.6
  - 0.1.7
  - 0.1.8
  - 0.1.9
  - 0.10.0
  - 0.10.1
  - 0.10.2
  - 0.10.3
  - 0.10.4
  - 0.10.5
  - 0.10.6
  - 0.10.7
  - 0.11.0
  - 0.11.1
  - 0.11.2
  - 0.11.3
  - 0.11.4
  - 0.12.0
  - 0.12.0b1
  - 0.12.0b2
  - 0.12.0b3
  - 0.12.1
  - 0.12.10
  - 0.12.11
  - 0.12.12
  - 0.12.13
  - 0.12.2
  - 0.12.3
  - 0.12.4
  - 0.12.5
  - 0.12.6
  - 0.12.7
  - 0.12.8
  - 0.12.9
  - 0.13.0
  - 0.13.1
  - 0.13.2
  - 0.13.3
  - 0.13.4
  - 0.13.5
  - 0.13.6
  - 0.13.7
  - 0.13.8
  - 0.14.0
  - 0.14.1
  - 0.14.2
  - 0.15.0
  - 0.16.0
  - 0.17.0
  - 0.17.1
  - 0.18.0
  - 0.19.0
  - 0.19.1
  - 0.2.0
  - 0.2.1
  - 0.2.2
  - 0.2.3
  - 0.20.0
  - 0.20.1
  - 0.20.2
  - 0.20.3
  - 0.20.4
  - 0.21.0
  - 0.22.0
  - 0.23.0
  - 0.23.1
  - 0.24.0
  - 0.25.0
  - 0.26.0
  - 0.26.0.post1
  - 0.26.1
  - 0.27.0
  - 0.28.0
  - 0.29.0
  - 0.3.0
  - 0.3.1
  - 0.3.2
  - 0.3.3
  - 0.3.4
  - 0.3.5
  - 0.3.6
  - 0.3.7
  - 0.30.0
  - 0.31.0
  - 0.31.1
  - 0.32.0
  - 0.32.0.post1
  - 0.33.0
  - 0.34.0
  - 0.35.0
  - 0.35.1
  - 0.36.0
  - 0.36.1
  - 0.36.2
  - 0.36.3
  - 0.37.0
  - 0.37.1
  - 0.37.2
  - 0.38.0
  - 0.38.1
  - 0.38.2
  - 0.38.3
  - 0.38.4
  - 0.38.5
  - 0.38.6
  - 0.39.0
  - 0.39.1
  - 0.39.2
  - 0.4.0
  - 0.4.1
  - 0.4.2
  - 0.40.0
  - 0.41.0
  - 0.41.1
  - 0.41.2
  - 0.41.3
  - 0.42.0
  - 0.43.0
  - 0.44.0
  - 0.45.0
  - 0.45.1
  - 0.45.2
  - 0.45.3
  - 0.46.0
  - 0.46.1
  - 0.46.2
  - 0.47.0
  - 0.47.1
  - 0.5.0
  - 0.5.1
  - 0.5.2
  - 0.5.3
  - 0.5.4
  - 0.5.5
  - 0.6.0
  - 0.6.1
  - 0.6.2
  - 0.6.3
  - 0.7.0
  - 0.7.1
  - 0.7.2
  - 0.7.3
  - 0.7.4
  - 0.8.0
  - 0.8.1
  - 0.8.2
  - 0.8.3
  - 0.8.4
  - 0.8.5
  - 0.8.6
  - 0.8.7
  - 0.8.8
  - 0.9.0
  - 0.9.1
  - 0.9.10
  - 0.9.11
  - 0.9.2
  - 0.9.3
  - 0.9.4
  - 0.9.5
  - 0.9.6
  - 0.9.7
  - 0.9.8
  - 0.9.9
references:
- type: WEB
  url: https://github.com/encode/starlette/security/advisories/GHSA-2c2j-9gv5-cj73
- type: ADVISORY
  url: https://nvd.nist.gov/vuln/detail/CVE-2025-54121
- type: FIX
  url: https://github.com/encode/starlette/commit/9f7ec2eb512fcc3fe90b43cb9dd9e1d08696bec1
- type: PACKAGE
  url: https://github.com/encode/starlette
- type: WEB
  url: "https://github.com/encode/starlette/blob/fa5355442753f794965ae1af0f87f9fec1b9a3de/starlette/datastructures.py#L436C5-L447C14"
- type: WEB
  url: "https://github.com/encode/starlette/discussions/2927#discussioncomment-13721403"
- type: PACKAGE
  url: https://pypi.org/project/starlette
- type: ADVISORY
  url: https://github.com/advisories/GHSA-2c2j-9gv5-cj73
severity:
- type: CVSS_V3
  score: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L