A spy who works for two mutually antagonistic countries WordNet 3.0 & Open English WordNet

In the field of counterintelligence, a double agent is an employee of a secret intelligence service for one country, whose primary purpose is to spy on a target organization of another country, but who is now spying on their own country's organization for the target organization. Wikipedia

Spy for one party who poses as a spy for the other. Wikipedia Disambiguation

Profession Wikidata

A person who is active in an Organisation or similar who is actively and secretly sharing some or all of its internal and confidential informations with another organisation or group of people most often targetting quite opposite goals. OmegaWiki

A spy who pretends to work for one side, when they are actually working for or passing information to the other. Wiktionary

Spy. Wiktionary (translation)