As an employee I signed a waiver to release my background information to my employer. My employer then freely shared the entire contents of my background check with a client of the company. I gave no consent and had no knowledge of the information share until after it was shared. Is this legal in California?

How do you know for a fact it was actually shared? And was this "Client" a second company they were under contract to do work for?

More info needed, where you doing work for or in the business of the Client?