Not sure if I fully agree with the term "rootkit" for this but...Rootkit is technology that hides software from the user and security software. This kind of technology is normally used by malware authors that want their presence to remain undetected in the system as long as possible. DRM software is not malicious but it has other reasons for hiding from the user. DRM software restricts the user's ability to make copies of a record and for that reason uses technology that prevents removal and modification of the software.
Sony BMG is currently using a rootkit-based DRM system on some CD records sold in USA. As far as we know, this system has been in use since March 2005. We've made some test purchases for Sony BMG records from Amazon.com and can confirm that they contained this technology.
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More: http://www.sysinternals.com/blog/2005/1 ... ights.html