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VIRUSES, WORMS, TROJANS, BOMBS

incident was reported in 1987 in Germany. On the afternoon of 9 December several students at the University of Clausthal-Zellerfeld, just south of Hannover, logged in to their computers and found that they had received electronic mail in the form of a file called Christmas. On reading the file they saw the message ‘Let this exec run and enjoy yourself!’ followed by a small drawing of a Christmas tree, crudely represented by asterisks. An ‘exec’ is an executable file, or program, and the suggestion was that if they ran the program, a large Christmas tree would appear on the computer screens. By the side of the small drawing was the greeting: ‘A very merry Christmas and best wishes for the next year.’ Underneath the drawing was a further message, in broken English: ‘Browsing this file is no fun at all just type “Christmas”.’ There followed some seventy lines of instructions to the computer. The students could recognize that these instructions were written in an easy-to-use programming language that was available on their IBM mainframe, but few could comprehend what the program was designed to do.

Most of the students decided to give the program a try, typed in ‘Christmas’ and were duly rewarded with a large drawing of a Christmas tree. Typically, they then deleted the file. The next time they logged in to their computers, however, they found they had received more copies of the Christmas file, and so had many other computer users at the university. What no one had realized was that as well as drawing a Christmas tree, the program had been reading the files containing the students’ electronic address books with the details of their other regular contacts on the IBM mainframe computer. The program then sent a copy of itself to all the other names that it could find. It was an electronic chain letter. Each time the program was run it could trigger fifty or a hundred or even more copies of itself, depending on the size of each user's electronic address book.

The unidentified student who playfully introduced the Christmas file into the electronic mail system had probably visualized a little local fun. He hadn’t realized that some of the university’s computer users have electronic addresses outside Clausthal-Zellerfeld linked by EARNet, the European Academic Research Network. Or that when copies of the file started whizzing round EARNet they would then find their way on to BitNet – an academic computer network linking

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