white hat hacker
n. A hacker who, upon discovering a vulnerability in a computer system, alerts the system vendor to the problem.

Example Citation:
"Mell says the attack scripts are posted on hacker Web sites by other hackers, by disgruntled systems administrators trying to draw attention, and eventually patches, to holes in their systems, and by 'white hat' hackers seeking to alert the computer security industry to vulnerabilities."
—Michael E. Ruane, "New Computer Technology Makes Hacking a Snap," The Washington Post, March 10, 1999

There are also black hat hackers who exploit system security breaches for nefarious ends, and gray hat hackers who supply information about a security issue both to the vendor and to crackers (hackers who have succumbed to the dark side of the Force).

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