The source code of one of the world's most popular free security tools will no longer be available to all, its creator has announced, saying the software's open-source license was fueling competition.

Renaud Deraison, the primary author of the Nessus vulnerability scanner, broke the news in a message to the software's e-mail list Wednesday. "Nessus 3 will be available free of charge...but will not be released under the GPL," or General Public License, Deraison wrote. Nessus, which Deraison says is used by 75,000 organizations worldwide, scans networks for vulnerabilities.

The developer, who has been working on the product since at least 1998, said commercial pressures facing Tenable Network Security, the company he started in 2002 around Nessus, was forcing him to stop making the software's source code available.

"A number of companies are using the source code against us, by selling or renting appliances, thus exploiting a loophole in the GPL," he wrote in a later e-mail, justifying his decision. "So in that regard, we have been fueling our competition, and we want to put an end to that. Nessus 3 contains an improved engine, and we don't want our competition to claim to have improved 'their' scanner."