Purloining

He made some money on the side by purloining secrets from his employer and selling them to the competition.

c. 1613–1621, Francis Bacon, The judicial charge upon the commission of Oyer and Terminer held for the verge of the Court […] I must require you to use diligence in presenting especially those purloinings and imbezlements, which are of plate, vessel, or whatsoever within the King's house.