Is there a public exploit for CVE-2025-54121?

No public exploit for CVE-2025-54121 was found in Exploit-DB, Metasploit, Nuclei templates or public proof-of-concept repositories. That is an absence of evidence rather than proof that none exists.

How severe is CVE-2025-54121?

CVE-2025-54121 scores CVSS 5.3 (v3.1), rated medium. EPSS puts the chance of exploitation in the next 30 days at 0.66%.

What does CVE-2025-54121 affect?

CVE-2025-54121 affects encode starlette.

5.3 Medium

Starlette is a lightweight ASGI (Asynchronous Server Gateway Interface) framework/toolkit, designed for building async web services in…

CVE-2025-54121

Summary When 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. 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](https://github.com/encode/starlette/blob/fa5355442753f794965ae1af0f87f9fec1b9a3de/starlette/datastructures.py#L436C5-L447C14)) 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:…

Assessment

CVSS: 5.3 (v3.1)
Severity: Medium
EPSS: 0.66% chance of exploitation in the next 30 days
Known exploited: Not listed in CISA KEV
Exploit availability: Nothing found in Exploit-DB, Metasploit, Nuclei or public PoC repositories
Published: 21 July 2025
Last updated: 17 June 2026
Affected: encode · starlette · starlette
Weakness: CWE-770
Assigner: security-advisories@github.com

CVSS vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

At the source

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