Ernst & Young sited a mole discovered in a bank, planted by the syndicate.

In Singapore, this type of activity has lead to "integrity interviews" as an effort to hire "good guys" instead of bad :-)

While hiring good people and doing diligence via background checks - county, state, multi-state, federal, civil, criminal, credit reports, lifestyle polygraphs, counter intelligence polygraphs and the like might help, don't forget, this doesn't guarantee anything. Consider these three very high profile cases.

• Aldrich H. Ames – began working for the CIA in 1962. Partly because of pressure from a divorce and financial difficulties he began spying for the Soviet Union more than two decades later in 1985. He is currently serving life without parole. (Movie – Traitor Within)
• Robert P. Hansen – began working for the FBI in 1976. Only three years later in 1979 he began spying for the Soviets and later the Russians. He wasn’t arrested until 2001. He is now serving life without parole. (Movie – Breach)
• Ronald W. Pelton – began working for the NSA in the early 1960s. He retired in 1980; in 1984 he began facing financial difficulties and sold secrets to the KGB while vacationing in Vienna Austria. He is serving three consecutive life sentences.

Not only do these organizations do detailed background checks, but they also follow close need-to-know policies, have various levels of clearance, and re-assess employees throughout their careers.