WordNet records no synonym and no antonym for loophole. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
an ambiguity (especially one in the text of a law or contract) that makes it possible to evade a difficulty or obligation
a small hole in a fortified wall; for observation or discharging weapons
Synonyms, antonyms and the broader and narrower relations for loophole come from Princeton University’s WordNet. Full sources and licences.