What’s Vulnerable › Vulnerabilities

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

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.7% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2025-07-21 · updated 2026-06-17
• Weakness: CWE-770

Description

Starlette is a lightweight ASGI (Asynchronous Server Gateway Interface) framework/toolkit, designed for building async web services in Python. In versions 0.47.1 and below, 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 the application can't accept new connections. The UploadFile code has a minor bug where instead of just checking for self._in_memory, the logic should also check if the additional bytes will cause a rollover. The vulnerability is fixed in version 0.47.2.

Affected products

• Kludex starlette: before 0.47.2

References

• https://github.com/encode/starlette/blob/fa5355442753f794965ae1af0f87f9fec1b9a3de/starlette/datastructures.py#L436C5-L447C14
• https://github.com/encode/starlette/commit/9f7ec2eb512fcc3fe90b43cb9dd9e1d08696bec1
• https://github.com/encode/starlette/discussions/2927#discussioncomment-13721403
• https://github.com/encode/starlette/security/advisories/GHSA-2c2j-9gv5-cj73

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