If by “leak” you mean the ink cartridge itself has ink dripping out, then the printer needs a new cartridge.
However, if the black ink smudges on printed pages, then it could be one of two things:
# The printer’s fusing assembly needs replacing. The fusing assembly uses heat and pressure to “fuse” the ink to the paper.
# The printer paper that is currently being used is not the specific type of paper that the printer manufacturer designated for use with that printer. The “wrong” type of paper can cause poor contact between the paper and the ink, causing it to leak off the paper. The printer’s manual should list the type of printer paper that the manufacturer wants.