A white hat (or a white-hat hacker, a whitehat) is an ethical security hacker. Under the owner's consent, white-hat hackers deliberately hack software or system with the aim of identifying any vulnerabilities or security issues it has, helping to reinforce it from black hat hackers.

The white hat is contrasted with the black hat, a malicious hacker; this definitional dichotomy comes from Western films, where heroic and antagonistic cowboys might traditionally wear a white and a black hat, respectively. There is a third kind of hacker known as a grey hat who hacks with good intentions but at times without permission or proper consent.

White-hat hackers may also work in teams called "sneakers", hacker clubs, red teams, or tiger teams.

The modern contrast between white hat and black hat derives from the convention in Western films in which heroic characters were associated with white hats and villains with black hats. By the mid-1960s, white hat was being used more generally in American English to mean a person perceived as one of the "good guys" or as being on the side of right.

The computing sense developed from this wider moral contrast. The Oxford English Dictionary records white hat in computing slang from 1990, defining it as a person who engages in computer hacking for benign or altruistic purposes, especially to test security systems and prevent illegal acts. The related expression white-hat hacker was in use by the late 1990s, and by the early 2000s was being used for security testers contracted to probe networks and systems for weaknesses.

Interviews of staff in the UK in 2011 suggest that ethical hackers working for companies have skills around social engineering, mobile tech, and social networking.

In professional employment, the work of white-hat hackers substantially overlaps with penetration testing and ethical security testing with the ethical hacker most closely covering the role of a penetration tester, simulating the attacks used by malicious hackers in order to understand how systems can be defended.

White-hat roles may be filled by in-house staff or by third-party specialists contracted to test an organisation's security. In this setting, the distinction from malicious hacking is not primarily technical method, but authorisation, scope and professional discipline: white hats use attacker-like techniques within agreed legal and ethical boundaries.

The penetration testing or ethical security testing industry had become organised around professional organisations, recognised qualifications and structured development routes, while also stressing that practitioners must not exceed the boundaries agreed with the client.

A wide variety of security assessment tools are available to assist with penetration testing, including free-of-charge, free software, and commercial software.

In July 2021, the Chinese government moved from a system of voluntary reporting to one of legally mandating that all white hat hackers first report any vulnerabilities to the government before taking any further steps to address the vulnerability or make it known to the public. Commentators described the change as creating a "dual purpose" in which white hat activity also serves the country's intelligence agencies.