A back door into the Unix operating system that was placed there by Ken Thompson, who designed Unix while working for Bell Labs in the 1980s. The Ken Thompson Hack (KTH) allowed him to log in to Unix while it was running. However, the hack was never in the Unix source code itself. The code was cleverly hidden in the C compiler and inserted into the Unix executable when it detected it was compiling the Unix OS. See
Unix,
B,
C and
back door.