A Canadian computer whiz who hacked into some of the world's most prominent websites when he was just 15 years old revealed in a new tell-all book that he was motivated by a so-called "hacker war."

The hacker, known as Mafiaboy, said that back in 2000 when he cracked sites like Yahoo and eBay, computer hackers frequented chat rooms where they were constantly challenging each other.

"Obviously, as you can see, it got a bit out of hand," Mafiaboy, whose real name is Michael Calce, said Thursday in an interview on CTV's Canada AM.

In 2000, a hacker from Montreal known only as Mafiaboy managed to shut down five websites, including Yahoo, eBay and CNN.

After a manhunt, the FBI and the RCMP arrested Calce, who pleaded guilty to more than 50 charges and spent eight months in a youth detention centre.

Now 24, he tells his story in Mafiaboy: How I Cracked the Internet and Why It's Still Broken.

Calce said he didn't realize the ramifications of what he was doing, including the financial damages to the companies whose websites he hacked. The FBI estimated the damages at several million dollars.

"In a technical sense I knew what I was doing on the computer but I didn't realize the full gravity of the crime that I was committing. You don't think about all of the repercussions that are involved," Calce said Thursday in an interview on CTV's Canada AM.

"At 15 years old you can't take all these factors into account."

Calce got his first computer when he was six years old and first went online when he was nine.

He said it was "surprisingly very easy" to hack into the sites and not much has changed since he committed his crimes.

In his book, Calce says hacking incidents are growing by the thousands and hackers are now no longer just competing with each other.

When he was hacking, he was not seeking financial gain, Calce said.

Now hackers are trying to make money by accessing databases that contain the personal information of average Internet users, Calce said.

Calce is now using his expertise to help companies protect their computer systems against hackers.

"I'm more focused on the aspect of helping people rather than financial gain. It's also a profession so sure I can make some money but it's more the objective of helping people out there."