Hackers have earned themselves a bad reputation. They are infamously known to gain unauthorized access to systems and networks to commit crimes. But there are those hackers who have helped shape our cybersecurity landscapes forever. They are brilliant minds who have donned both black hats and white hats and explored methods to improve defensive capabilities. Here’s a look at four of the best and most famous hackers.

1.     Kevin Mitnick

The US Department of Justice referred to him as the “most wanted computer criminal in US history.” He went on a hacking spree that involved breaching over 40 major corporations.

Today, Mitnick is a high-profile security consultant considered the world’s authority on social engineering, security awareness training, and hacking. He is a best-selling author, and his books are a mandatory read for all security professionals.

2.     George Francis Hotz

Hotz, a.k.a. GeoHot, is an American software engineer and security hacker. He is known as the first person ever to unlock the SIM lock on an iPhone. He has successfully breached and reverse engineered PlayStation 3, so players could run their code on locked hardware.

Since his fraudulent activities, he has made a meaningful comeback working for Facebook and then Google, and then with a startup called Vicarious. Today he is the President of Comma.ai.

3.     Barnaby Michael Douglas Jack

Barnaby Jack was a New Zealand programmer and hacker. He is best known for his presentation at a 2010 Black Hat computer security conference where he exploited how ATMs could be forced to dispense currency. He was also infamously known to have been able to hack into medical devices like pacemakers.

Barnaby Jack was considered one of the best computer security professionals and a leader in the financial and medical security fields. A 2012 testimony led to the US Food and Drug Administration (FDA) changing wireless medical device regulations. According to Trey Ford, General Manager of Black Hat, the life and work of Jack were both “legendary and irreplaceable.”

4.     Peiter C. Zatko

Peiter was better known as Mudge. He was the most well-known member of L0pht – a collective hacker group. In 1998, Mudge and other members of the L0pht warned a Senate committee that computer hardware, software, and networks that linked computers were not safe. They said the Internet was not the place to be for computer security. Sadly, nobody heeded their warnings. After the first significant internet attacks, he was invited to meet with President Bill Clinton at a security summit. He is one of the first hackers to reach out to the government and industry to help create safer and better cybersecurity. Today, he works as the Head of Security at Twitter.