Christmas Tree EXEC was the first widely disruptive computer worm

Computer worm

A computer worm is a self-replicating malware computer program, which uses a computer network to send copies of itself to other nodes and it may do so without any user intervention. This is due to security shortcomings on the target computer. Unlike a computer virus, it does not need to attach...

, which paralyzed several international computer networks in December 1987.

Written by a student at the Clausthal University of Technology

Clausthal University of Technology

The Clausthal University of Technology is an institute of technology in Clausthal-Zellerfeld, Lower Saxony, Germany...

 in the REXX

REXX

REXX is an interpreted programming language that was developed at IBM. It is a structured high-level programming language that was designed to be both easy to learn and easy to read...

 scripting language, it drew a crude Christmas tree as text graphics

ASCII art

ASCII art is a graphic design technique that uses computers for presentation and consists of pictures pieced together from the 95 printable characters defined by the ASCII Standard from 1963 and ASCII compliant character sets with proprietary extended characters...

, 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

BITNET

BITNET was a cooperative USA university network founded in 1981 by Ira Fuchs at the City University of New York and Greydon Freeman at Yale University...

, and IBM's worldwide VNET. On all of these systems it caused massive disruption.

Its core mechanism was essentially the same as the ILOVEYOU

ILOVEYOU

ILOVEYOU, also known as Love Letter, is a computer worm that successfully attacked tens of millions of computers in 2000 when it was sent as an attachment to a user with the text "ILOVEYOU" in the subject line. The worm arrived e-mail on and after May 4, 2000 with the simple subject of "ILOVEYOU"...

 worm of 2000 - although running on mainframes rather than PCs, spreading over a different network, and scripted using REXX

REXX

REXX is an interpreted programming language that was developed at IBM. It is a structured high-level programming language that was designed to be both easy to learn and easy to read...

 rather than VBScript

VBScript

VBScript is an Active Scripting language developed by Microsoft that is modeled on Visual Basic. It is designed as a “lightweight” language with a fast interpreter for use in a wide variety of Microsoft environments...

.

Trivia

The name is sometimes written "CHRISTMAS EXEC" because the script file is an "EXEC" (executable), and the file concerned was in fact named CHRISTMA, as IBM systems of the day only supported 8 character filenames. The user was prompted to: "...just type CHRISTMAS..." - and this in fact launched the "worm".