PYSEC-2026-1851

Denial of service (DoS) via deformation `multipart/form-data` boundary

### Summary When parsing form data, `python-multipart` skips line breaks (CR `\r` or LF `\n`) in front of the first boundary and any tailing bytes after the last boundary. This happens one byte at a time and emits a log event each time, which may cause excessive logging for certain inputs. An attacker could abuse this by sending a malicious request with lots of data before the first or after the last boundary, causing high CPU load and stalling the processing thread for a significant amount of time. In case of ASGI application, this could stall the event loop and prevent other requests from being processed, resulting in a denial of service (DoS). ### Impact Applications that use `python-multipart` to parse form data (or use frameworks that do so) are affected. ### Original Report This security issue was reported by: - GitHub security advisory in Starlette on October 30 by @Startr4ck - Email to `python-multipart` maintainer on October 3 by @mnqazi