PortHack.exe is an executable in Hacknet that is used to gain administrator access to a computer remotely using open ports. It is already dispatched with the Hacknet OS by default on your system and is rarely found on other machines. It can be run by typing 'PortHack' into the command terminal.
In the final mission of Hacknet Bit informs you that Hacknet once was a standalone program but is now linked to a central computer node called PortHack.Heart. You have to shutdown the heart by using PortHack on itself, killing the heart process. Once completed the epilogue starts and you hear an audio log of Bit explaining that you did it.