A denial of service (DoS) vulnerability exists in FastAPI via the python-multipart library when parsing multipart/form-data boundaries. The issue arises because python-multipart skips line breaks one byte at a time before the first boundary and after the last boundary, emitting log events for each byte. An attacker can exploit this by sending specially crafted requests with excessive data around boundaries, causing high CPU usage and stalling the processing thread or event loop, leading to DoS. This affects FastAPI versions from 0.111.0 up to but not including 0.115.5_1. A fix is available in version 0.115.5_1.