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:
python-multipart
installed andrequest.form()
UploadFile
parameters, which in turn calls request.form()
.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=1000
max_fields=1000
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.
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.
{ "nvd_published_at": null, "cwe_ids": [ "CWE-400" ], "severity": "HIGH", "github_reviewed": true, "github_reviewed_at": "2023-02-14T21:31:28Z" }