The Cuckoo's Egg

by Clifford Stoll

Richard Stallman, a free-lance computer programmer, loudly proclaimed that information should be free. His software, which he gives away for free, is brilliantly conceived, elegantly written, and addictive. Over the past decade Stallman created a powerful editing program called Gnu-Emacs. But Gnu’s much more than just a text editor. It’s easy to customize to your personal preferences. It’s a foundation upon which other programs can be built. It even has its own mail facility built in. Naturally, our physicists demanded Gnu; with an eye to selling more computing cycles, we installed it happily. Just one problem: there’s a bug in that software. In the way it was installed on our Unix computer, the Gnu-Emacs editor lets you forward a mail file from your own directory to anyone else in an unusual way. It doesn’t check to see who’s receiving it, or even whether they want the file. It just renames the file and changes its ownership label. You’ve just transferred ownership of the file from you to me. No problem to send a file from your area to mine. But you’d better not be able to move a file into the protected systems area: only the system manager is allowed there. Stallman’s software had better make sure this can’t happen. Gnu didn’t check. It let anyone move a file into protected systems space. The hacker knew this; we didn’t. The hacker used Gnu to swap his special atrun file for the system’s legitimate version. Five minutes later, the system hatched his egg, and he held the ...