There is a stack overflow vulnerability in this challenge, by which you can leak read@GOT, find glibc base address, and jump to execve found by one gadget using return oriented programming (ROP) technique.
return-to-csu: A New Method to Bypass 64-bit Linux ASLR BlackHat talk is a must-read (https://www.blackhat.com/docs/asia-18/asia-18-Marco-return-to-csu-a-new-method-to-bypass-the-64-bit-Linux-ASLR.pdf).