Sven Hoppe/picture alliance via Getty Images This story is available exclusively to Insider subscribers. Become an Insider and start reading now. Researchers warned for years Facebook's phone number lookup tools were vulnerable to abuse. Facebook said it fixed the issue in 2019. But by then, more than 500 million users' data had been stolen. Researchers also said they found a near-identical problem in WhatsApp soon after, but Facebook initially said it wasn't an issue. Facebook fixed a security vulnerability in August 2019 that had allowed hackers to scrape more than 500 million users' data from its app. But the company had been warned about the risks of the tools than enabled the data exfiltration years before it finally took action.