CVE-2025-54121 Starlette has possible denial-of-service vector when parsing large files in multipart forms
Attackers can cause denial of service by uploading large files, blocking the main thread and preventing new connections in Starlette applications.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-54121?
CVE-2025-54121 is a medium-severity denial of service vulnerability in encode starlette, published on 21 Jul 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-770
Which versions are affected by CVE-2025-54121?
3 open source packages ship an affected version, including starlette.
| Package | Affected | Fixed in |
|---|---|---|
starlette PyPI | All versions | 0.47.2 |
starlette PyPI | All versions | 0.47.2 |
starlette PyPI | All versions | 0.47.2 |
Linux distribution packages (2)
python-starlette openSUSE:Leap 15.6 | All versions | 0.35.1-150600.3.6.1 |
python-starlette openSUSE:Tumbleweed | All versions | 0.47.2-1.1 |
How do I fix CVE-2025-54121?
Upgrade starlette to 0.47.2 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2025-54121 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2025-54121 exists.
Sources
Also known as CLEANSTART-2026-YZ00893, CLSA-2025-1764115957, CLSA-2025-1764288754, DEBIAN-CVE-2025-54121, GHSA-2c2j-9gv5-cj73, MINI-mfg9-9m76-wmxh and 5 more.