Rating:

The binary is a simple HTTP server.
It has a buffer overflow vulnerability when getting the URL.
By causing an error during sys_write we can call an arbitrary system call with rdi under our control.
Thus, we can run a command by calling sys_execve with the arguments(rsi) set properly.

Original writeup (https://ptr-yudai.hatenablog.com/entry/2019/04/25/141422#Binary-180pts-Server).