RCTF 2018 - stringer challenge contains off-by-one and double free vulnerabilities. Lesson learned is that if the chunk being allocated is MMAPED, the content will not be zero out when using calloc.
So, by using off-by-one attack, we can set IS_MMAPED bit of the target chunk in order to leak a libc address, and then launch the fastbin attack (https://github.com/shellphish/how2heap/blob/master/fastbin_dup_into_stack.c) by using double free vulnerability in order to overwrite __malloc_hook.
This is a good challenge to understand how to exploit x86_64 binaries with Full RELRO, Canary, NX, PIE, and ASLR enabled.