Ken Thompson's "cc hack"
Presented in the journal, Communication of the ACM, Vol. 27, No. 8, August 1984, in a paper entitled "Reflections on Trusting Trust".
Ken Thompson, co-author of UNIX, recounted a story of how he created a version of the C compiler that, when presented with the source code for the "login" program, would automatically compile in a backdoor to allow him entry to the system. This is only half the story, though. In order to hide this trojan horse, Ken also added to this version of "cc" the ability to recognize if it was recompiling itself to make sure that the newly compiled C compiler contained both the "login" backdoor, and the code to insert both trojans into a newly compiled C compiler. In this way, the source code for the C compiler would never show that these trojans existed.