For two years, Uber reportedly ran a program that exploited a security vulnerability in the software of rival ride-hailing company Lyft to track and spy on Lyft drivers, according to a report from the Information.

Uber’s corporate espionage program was referred to as “Hell”—a play on the company’s program for tracking its own drivers and riders, known as “God View” or “Heaven” —and was only accessible to top level executives and a select group of data scientists.

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The Uber Hell program reportedly started when Uber began creating fake rider accounts on Lyft. It used those accounts to spoof riders on its rival’s platform. The fake passengers would provide Uber with a view of the eight closest drivers, so it expanded its spoofing process to create a city-wide grid that allowed it to keep an eye on the movements of Lyft drivers throughout a city.

Through the grid, Uber noted Lyft drivers have unique numbered IDs that never change—a process that is unlike Uber’s system, which rotates tokens for drivers. Because Lyft drivers have a permanent identifier, Uber was able to track the driver’s habits.

Uber began exploiting this ability to identify drivers who were “double-apping,” or driving for both Lyft and Uber—a relatively common practice for full-time drivers using ridehailing apps.

Once the company identified drivers working for its competitor—about 60 percent of Lyft drivers were also working for Uber—it began sending them more riders and offering bonuses for performing a certain number of rides in a week in order to entice the drivers to work exclusively for Uber.

The program, which was apparently approved by Uber CEO Travis Kalanick, operated from 2014 to 2016, and stopped only because Lyft’s expansion into new cities would have made the bonus payouts too costly to continue offering.

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Uber has not spoken on the record about the alleged program yet, but a spokesperson for Lyft told the Information, “We are in a competitive industry. However, if true, these allegations are very concerning."

The Information also reported several law firms that worked with Uber in the past advised the “Hell” program could present the company with a number of problems, including breach of contract, unfair business practices, misappropriation of trade secrets and violations of the Computer Fraud and Abuse Act.