In this hypothetical case, extra functionality was written into the software tools (a compiler) used to create software. This was designed to inject a password backdoor into the logon software of the Unix operating system. Anyone who knew of the flaw could then login as anybody without knowing their password. But the tools are also used to build themselves, and the really clever bit is that the extra functionality also adds itself when it builds the tools. Once this is done, the flaw is invisible.