Guess the flag (vuln 100)

There is given service raised at vuln1.quals.ructf.org:16712 and it's ELF 32-bit executable source file.

At first I tried to reverse executable but i'm not a reverse-engeneering-guy so i got nothing :)

Because of task's cost is 100 and I thought than it can't be difficult I connect to vuln1.quals.ructf.org:16712 and started to brute inputs. And I was surprised that after several attempts I got flag RUCTF_f4205156a73b7bd143ab06e7722e3c81f72b8429 with "\" as input string :)