This exploit steals the private key of the Proftpd server without any address leakage of the randomized key. To mount this exploit, attackers send server requests to make the program do necessary calculations and memory dereferences for attackers. Finally the key's content is send to attackers.
This exploit modifies the code page of the Proftpd server. Specifically, it changes one "ret" instruction with a "int3" instruction. The result is that the programm sends "TRAP" singal to the system.