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Common name MDEF
Technical name MDEF
Aliases Garfield
Top Cat
Family Unknown
Classification Virus
Type Macintosh
Subtype Nuisance
Isolation May 1990
Point of isolation Ithaca, New York
Point of origin Ithaca, New York
Author(s) Same author as CDEF

MDEF was a computer virus affecting Macintosh machines. There are four known strains. The first, MDEF A (aka Garfield), was discovered in May 1990. Strains B (aka Top Cat), C, and D were discovered in August 1990, October 1990, and January 1991, respectively.

MDEF A, B, and C can infect application files and system files, and sometimes document files as well. The D strain will infect only applications. None of the viruses were designed to do damage, but they often do. MDEF D can sometimes damage applications beyond repair.

Quick action by computer security personnel and the New York State Police resulted in identification of the author, a juvenile. This was the same person responsible for writing the CDEF virus.


  • Norstad, John. The Viruses. Disinfectant 3.7.1 ©1988-1997 Northwestern University.