Cap'n Proto is a data interchange format and capability-based RPC system. Prior to version 1.4.0, when using Transfer-Encoding: chunked, if a chunk's size was parsed to a value of 2^64 or larger, it would be truncated to a 64-bit integer. This could potentially enable HTTP request/response smuggling. The issue is related to the parsing of chunk sizes in HTTP requests when the `Transfer-Encoding` is set to `chunked`.