LAS VEGAS — The Web site of ManTech International, a $2.6 billion computer security company that won a major F.B.I. contract, sells its services this way:

“Whether an intrusion is conducted by a skilled outsider with criminal intent, an adolescent hacker seeking a thrill or a disgruntled employee bent on revenge or espionage, the potential risks to the organization are enormous.”

Last Friday, ManTech was that organization.

A band of Internet vigilantes calling itself Anonymous said it had sneaked into ManTech’s computers to demonstrate the company’s insecurity. The group released what it said were internal company documents and, in language that suggested the handiwork of an adolescent hacker seeking a thrill, taunted the company online: “It’s really good to know that you guys are taking care of protecting the United States from so-called cyber threats.”

ManTech is in good company. In recent months, several security firms and consultants have been hit by the very intruders they are hired to keep at bay.

Think of these companies as the new Pinkertons: Instead of taking on 19th-century outlaws in the Wild West, they are hired today to protect corporate and government data, including the most confidential intelligence information, across a vast virtual frontier. The string of embarrassing attacks on them demonstrates how vulnerable everyone is online, including those who are paid to be the protectors.