But there's another breed of hacker out there, one who works at foiling the efforts of the troublemakers. Unlike the hackers who attempt to break into corporate networks for sport and spying purposes, so-called ethical hackers typically hire themselves out to perform "vulnerability assessments" for clients — meaning they essentially break into the client's computer network with the client's consent in the interest of patching up security holes. . . .

Companies ranging in size from startups to International Business Machines Corp. have ethical-hacking teams. Computer-security services, including vulnerability assessments by ethical hackers and other services, was a $1.8-billion (U.S.) worldwide market last year, and is expected to grow at a compound annual rate of 28 per cent for the next three years, according to Gartner Inc., a market research firm in Stamford, Conn.
—Nick Wingfield, "It Takes a Hacker," The Wall Street Journal, March 11, 2002

Last year, a group of West German computer hobbyists successfully entered an international computer network belonging to the U.S. National Aeronautics and Space Administration. They were "ethical" hackers and didn't break anything. But still, they had the run of the place for at least three months.
—Michael Tucker, "A healthy paranoia," Computerworld, April 6, 1988