FastAPI is a web framework for building APIs with Python 3.8+ based on standard Python type hints. When using form data,
python-multipart uses a Regular Expression to parse the HTTP
Content-Type header, including options. An attacker could send a custom-made
Content-Type option that is very difficult for the RegEx to process, consuming CPU resources and stalling indefinitely (minutes or more) while holding the main event loop. This means that process can't handle any more requests. It's a ReDoS(Regular expression Denial of Service), it only applies to those reading form data, using
python-multipart. This vulnerability has been patched in version 0.109.0.