Allocation of Resources Without Limits or Throttling in starlette - CVE-2025-54121

Detailed vulnerability description

The vulnerability allows a remote attacker to perform a denial of service (DoS) attack.

The vulnerability exists due to 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. A remote attacker can the vulnerability and perform a denial of service (DoS) attack.

How to mitigate CVE-2025-54121

Sources

• 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
• https://github.com/Kludex/starlette/security/advisories/GHSA-2c2j-9gv5-cj73