Which is correct in German: ist gestanden, or hat gestanden?

Answer 1

hat gestanden, ist gestanden is used by some people in the southern part of Germany.

Answer 2

Both are correct.
With verbs of position (stehen, sitzen, liegen...) there are regional differences.
In the northern parts of Germany the "Perfekt" or "Vergangenheit" (present perfect) is formed with these verbs by using the appropriate form of "haben" while in southern Germany, Austria and Switzerland it is formed using "sein".
So these are regional variations but both forms ARE correct.

It is not true that only "hat gestanden" is correct and "only a few people in southern Germany" use "ist gestanden".
In general river Main (the one which runs through Frankfurt) is regarded to be the "sein - haben" border (north of it it is "haben", south of it it is "sein".

Some people even say it shows what is more important to people - to have or to be ;-)

Answer 3

Both are correct, as "stehen" can be conjugated with both "haben" and "sein".