We are given a statically linked binary with no PIE, and a helper client. We used the client to quickly deduce how to communicate with the server, by observing network traffic. Reversing the server turns out to be not too difficult; some reversing yields a function name xdr_string from an error message. From this we can deduced that Sun RPC XDR (eXternal Data Representation) objects are being used to communicate between the server and the client, and many of the important functions and structures can be filled in.
**Full Writeup:** [https://github.com/pwning/public-writeup/tree/master/rwctf2019/pwn_faX_senDeR](https://github.com/pwning/public-writeup/tree/master/rwctf2019/pwn_faX_senDeR)