"Also in his Turing Award lecture, he described how he had incorporated a backdoor security hole in the original UNIX C compiler. To do this, the C compiler recognized when it was recompiling itself and the UNIX login program. When it recompiled itself, it modified the compiler so the compiler backdoor was included. When it recompiled the UNIX login program, the login program would allow Thompson to always be able to log in using a fixed set of credentials."