CVE-2023-30798 is a high-severity (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in starlette. A fix is available for starlette — see the affected versions and patch details below.
MultipartParser DOS with too many fields or files in Starlette Framework
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2023-30798.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
starletteReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Impact
The MultipartParser using the package python-multipart accepts an unlimited number of multipart parts (form fields or files).
Processing too many parts results in high CPU usage and high memory usage, eventually leading to an <abbr title="out of memory">OOM</abbr> process kill.
This can be triggered by sending too many small form fields with no content, or too many empty files.
For this to take effect application code has to:
- Have
python-multipartinstalled and - call
request.form()- or via another framework like FastAPI, using form field parameters or
UploadFileparameters, which in turn callsrequest.form().
- or via another framework like FastAPI, using form field parameters or
Patches
The vulnerability is solved in Starlette 0.25.0 by making the maximum fields and files customizable and with a sensible default (1000).
Applications will be secure by just upgrading their Starlette version to 0.25.0 (or FastAPI to 0.92.0).
If application code needs to customize the new max field and file number, there are new request.form() parameters (with the default values):
max_files=1000max_fields=1000
Workarounds
Applications that don't install python-multipart or that don't use form fields are safe.
In older versions, it's also possible to instead of calling request.form() call request.stream() and parse the form data in internal code.
In most cases, the best solution is to upgrade the Starlette version.
References
This was reported in private by @das7pad via internal email. He also coordinated the fix across multiple frameworks and parsers.
The details about how multipart/form-data is structured and parsed are in the RFC 7578.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | starlette | all versions | 0.25.0pip install --upgrade 'starlette==0.25.0' |
Affected Products
starletteencodeDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for starlette, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update starlette to 0.25.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2023-30798 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Frequently Asked Questions
Is CVE-2023-30798 in your dependencies?
Find it across PyPI, including transitive dependencies.