I saw a little elf

ringzer0ctf

Task: a web page serves a nested-base64 Elf Message and a 3-second window to return a per-session secret; the payload is a byte-reversed x86-64 ELF. Solution: iteratively base64-decode until ELF magic appears, reverse the bytes, read the stack-built secret directly from mov immediates, and submit it in the same HTTP session.