Christmas Tree EXEC was the first widely disruptive computer worm, which paralyzed several international computer networks in December 1987. Written by a student at the Clausthal University of Technology in the REXX scripting language, it drew a crude Christmas tree as text graphics, then sent itself to each entry in the target's email contacts file. In this way it spread onto the European Academic Research Network (EARN), the BITNET, and IBM's worldwide VNET. On all of these systems it caused massive disruption.