PYSEC-2026-3039

### Summary

`python-multipart` has a denial of service vulnerability in multipart part header parsing. When parsing `multipart/form-data`, `MultipartParser` previously had no limit on the number of part headers or the size of an individual part header. An attacker could send a request with either many repeated headers without terminating the header block or a single very large header value, causing excessive CPU work before request rejection or completion.

### Impact

Applications that parse attacker-controlled `multipart/form-data` with affected versions of `python-multipart` can experience CPU exhaustion. ASGI applications using Starlette, FastAPI, or other frameworks that invoke `python-multipart` may have worker or event-loop delays while processing malicious upload requests.

### Details

The affected parser states are `HEADER_FIELD_START`, `HEADER_FIELD`, `HEADER_VALUE_START`, `HEADER_VALUE`, and `HEADER_VALUE_ALMOST_DONE`. The issue can be triggered by:

- A multipart part with an oversized individual header value. - A multipart part with many repeated header lines or an unterminated header block.

Both variants are addressed by enforcing default parser limits for maximum header count and maximum header size.

### Mitigation

Upgrade to `python-multipart` `0.0.27` or later.

If upgrading is not immediately possible, reduce exposure by enforcing request body size limits at the server, proxy, or framework layer. This is only a mitigation; affected versions of `python-multipart` still parse multipart part headers without the default header count and header size limits.