I've heard that H1b's and foreign contractors are the most common sources of security breaches at US firms.
"Contractors in Argentina and China were given "direct access to every row of data in every database" when they were hired by the Office of Personnel Management (OPM) to manage the personnel records of more than 14 million federal employees, a federal consultant told ArsTechnica."
http://www.businessinsider.com/the-us-agency-plundered-by-chinese-hackers-made-one-of-the-dumbest-security-moves-possible-2015-6