Trump and his regular unsecured discussions with Russian officials led him to be considered such a massive security risk to any intelligence operations in Russia. After Trump gave classified intelligence to Russians about Israeli counter-terrorism operations, there was mounting concern in the intelligence community about a CIA operative inside the Russian government. The information was withdrawn from Russia in 2017, despite having been in place for decades passing on information to the US government.

The New York Times – C.I.A. Informant Extracted From Russia Had Sent Secrets to U.S. for Decades
New York Magazine – U.S. Intelligence Had to Pull Spy From Russia Because Trump Is a Security Risk
CNN – Exclusive: US extracted top spy from inside Russia in 2017