This is the writeup from oak's challenge author. oak is a rev challenge coming with two binaries client, server, a log file access.log, and a script file run.sh. By inspecting the run.sh, we can know how the programs were run: Start the server binary. Execute the client binary with flag.jpg as the input file. However the flag.jpg is not included in the challenge files. With a quick decompiling on server (which is small), we can find that access.log is generated by server while processing client requests. Therefore, the goal is recovering flag.jpg from access.log. Explanation