Security researchers have identified a rootkit -- software used by hackers to hide their malicious code from anti-virus and anti-spyware defenses -- within the copy protection scheme Sony BMG Music Entertainment uses to prevent music CDs from being copied to computers.

The digital rights management (DRM) technology that Sony BMG uses limits the number of times a CD can be "ripped" to a computer. To prevent the DRM software from being easily circumvented, the copy protection's creator -- a U.K.-based company called First4Internet -- uses a rootkit to hide the DRM's files.

An independent researcher, Mark Russinovich, and the Helsinki-based F-Secure security firm, published details almost simultaneously on the DRM technology Sony BMG uses, and that technology's application of a rootkit.

Both stressed that rootkits are most commonly used by malicious code writers -- hackers -- and the use of it by a legitimate company such as Sony was alarming, they warned.

"Once the rootkit is there, there's no direct way to uninstall it," said Mikko Hyppnen, F-Secure's chief research officer, in an online brief. "The system is implemented in a way that makes it possible for viruses (or any other malicious program) to use the rootkit to hide themselves. too. This may lead to a situation where the virus remains undetected even if the user has got updated antivirus software installed."

Russinovich, who stumbled across the rootkit after a long investigation that involved a number of advanced PC forensic tools, agreed. "Not only had Sony put software on my system that uses techniques commonly used by malware to mask its presence, the software is poorly written and provides no means for uninstall."

In fact, when Russinovich tried to uninstall the DRM software, all he got for his trouble was a dead CD drive.

"Most users that stumble across the cloaked files with a RKR scan will cripple their computer if they attempt the obvious step of deleting the cloaked files," he said.

Removing the rootkit is so fraught with possibilities of calamity that F-Secure recommended users don't try it themselves. Instead, Hyppnen urged users to fill out a Sony BMG Web form and ask for instructions on how to remove the software. F-Secure has tested the resulting removal process -- which relies on the installation of an Internet Explorer ActiveX control -- and has confirmed it works.

According to one anti-spyware expert, Sony has no excuse for leaning on a rootkit to copy protect its content.

"Rootkits are always malicious," said Richard Stiennon, director of threat research for the Boulder, Colo.-based anti-spyware vendor Webroot. "There's no legitimate use of a rootkit, whose only purpose is to hide code from the operating system." Stiennon is intimately familiar with rootkits, since they're often by spyware writers to disguise some of their nastier work, like password keyloggers.

Stiennon's objection runs deeper than the rootkit code itself, however; he's also concerned that the copy protection software steps across another spyware line.

"The end user license agreement (EULA) doesn't mention any install [of a rootkit]," he said. "That likely makes it illegal in the U.K. and the EU, and in at least 10 states in the U.S. as well. Sony could be in a lot of trouble on this one.

"This is just the sort of thing that [anti-spyware advocates are] concerned about, that spyware laws, when written, will be too broad and won't take things like this into consideration," Stiennon added.

Even discounting Stiennon's concerns, however, Sony's use of a rootkit poses immediate danger, said F-Secure. In a technical description of the DRM software, the security vendor noted that "the hiding techniques can be abused by less technical malware authors to hide their backdoors and other tools."

All an attacker needs to do is name his files beginning with the same "$sys$" prefix used by the Sony CD copy protection files.

"It is very inappropriate for commercial software to use these techniques," said F-Secure.

Sony BMG did not immediately return a call for comment.