The Collaborative International Dictionary of English v.0.48:

magic bullet \magic bullet\ n. [From the notion of a bullet that
   will kill only the bacterium or disease it is intended for.]
   A drug or therapy or preventive that cures or prevents a
   disease, with only minimal side effects; as, there is no
   magic bullet against cancer.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

magic bullet
    n 1: a remedy (drug or therapy or preventive) that cures or
         prevents a disease; "there is no magic bullet against
         cancer"

The Free On-line Dictionary of Computing (30 December 2018):

magic bullet
silver bullet

    (Or "silver bullet" from vampire legends) A term
   widely used in software engineering for a supposed quick,
   simple cure for some problem.  E.g. "There's no silver bullet
   for this problem".

   (1999-01-13)