Idiom for magic object (or idea) that fixes everything

You may be thinking of magic bullet but panacea would also fit.

magic bullet noun
informal
a medicine or other remedy with advanced or highly specific properties:
there’s no magic bullet, and we should just try to eat as varied and well-balanced a diet as possible

panacea noun
a solution or remedy for all difficulties or diseases:
the panacea for all corporate ills
the time-honoured panacea, cod liver oil

In addition to "magic bullet" there is "silver bullet", particularly in the phrase "no silver bullet", to mean no quick or easy or complete fix for a problem.

See http://www.worldwidewords.org/qa/qa-sil1.htm for a comparison to "magic bullet".

This isn't a phrase, but there is the word panacea which the Oxford Dictionaries define as "a solution or remedy for all difficulties or diseases".

While the offered answers of panacea and magic bullet both seem to suit, there is also deus ex machina

noun [singular] literature /ˌdeɪəs eks ˈmɑkɪnɑ/ someone or something that solves a situation that seemed impossible to solve in a sudden and unlikely way, especially in a book, play, movie, etc.

This is largely literary, but can be used to describe an implausible solution in other cases.

It literally means god from a machine, referring to the common practice in bad theater of wrapping up a convoluted plot by having a god [usually Olympian] being lowered from by a crane or lifted through a trapdoor onto the stage and saving the beleaugered, issuing an edict or effecting a magical transformation.