The phrase "massive loophole" means a big gap or flaw in a rule or law that allows people to take advantage of it and avoid following the intended regulations or restrictions. Full definition
The plan creates a massive loophole with which ordinary people can evade taxes. (vox.com)
and the massive loopholes within it. (sbnation.com)
«This massive loophole must be closed in order to restore integrity to our elections and return the power of the political process to New Yorkers.» (briankavanagh.org)