The Free On-line Dictionary of Computing (19 January 2023):

cookie monster
cookie bear

    (From the children's TV program "Sesame Street")
   Any of a family of early (1970s) hacks reported on TOPS-10,
   ITS, Multics and elsewhere that would repeatedly demand "I
   WANT A COOKIE" until the victim typed "COOKIE".

   [Jargon File]

   (1997-02-12)


The Jargon File (version 4.4.7, 29 Dec 2003):

cookie monster
 n.

    [from the children's TV program Sesame Street] Any of a family of early
    (1970s) hacks reported on TOPS-10, ITS, Multics, and elsewhere that
    would lock up either the victim's terminal (on a timesharing machine) or
    the console (on a batch mainframe), repeatedly demanding “I WANT A
    COOKIE”. The required responses ranged in complexity from “COOKIE” through
    “HAVE A COOKIE” and upward. Folklorist Jan Brunvand (see FOAF) has
    described these programs as urban legends (implying they probably never
    existed) but they existed, all right, in several different versions. See
    also wabbit. Interestingly, the term cookie monster appears to be a 
    retcon; the original term was cookie bear.