a sly dog

Someone who, through cunning, devious, or dishonest means, achieves a fortuitous outcome. What a sly dog, he managed to get a copy of the test so he could memorize the answers ahead of time! He's a sly dog in business, cleverly negotiating deals with and between companies that would normally be in fierce competition with one another. Tabloid reporters are such sly dogs, spinning people's personal problems into sensationalist stories.