Tech workers at major U.S. companies are earning thousands of dollars by referring job candidates they’ve never met, creating an underground marketplace for employment referrals at firms like Microsoft and Nvidia, according to Bloomberg.

One tech worker cited in the report earned $30,000 in referral bonuses after recommending over 1,000 strangers to his employer over 18 months, resulting in more than six successful hires. While platforms like ReferralHub charge up to $50 per referral, Goldman Sachs and Google said such practices violate their policies. Google requires referrals to be based on personal knowledge of candidates.