Apple has modified its iTunes software to fix what it calls a "security hole" that allowed users to download music without the firm's proprietary copy protection system.

Norwegian hacker Jon Lech Johansen (also known as DVD Jon), Travis Watkins and Cody Brocious recently released PyMusique, which allowed users to listen to iTunes downloads on media players other than the iPod.

Apple stated that anyone looking to buy its music must use iTunes 4.7.1, which blocks the use of PyMusique and automatically adds the FairPlay copy protection to downloads.

Previous versions of iTunes added the protection after the download was complete, and PyMusique can block that installation.

"The security hole in the iTunes Music Store which was recently exploited has been closed, and as a consequence the Music Store will now sell music only to customers using iTunes version 4.7.1," Apple said in a statement.

This is not the first run-in Johansen has had with Apple. He has previously developed code called QTFairUse which strips out copy protection from QuickTime movies, and JustePort that did the same for file transfers over Apple's AirPort Express wireless technology.