First of all as you are born under the lagan Sagittarius, Jupiter is your Lagan lord. Your Lagan lord cannot be malefic. Yes it is placed in the 12th house at 5 degrees 15 minutes in Anuradha Nakshatra. Being in the 12th house, Jupiter becomes weak. Therefore Jupiter is weak, but not malefic.
Blessings,
Navneet Khanna
Astrologer