How Long Have Rootkits Been Around?
As we noted previously, rootkits are not a new concept. In fact, many of the methods used in modern rootkits are the same methods used in viruses in the 1980s—for example, modifying key system tables, memory, and program logic. In the late 1980s, a virus might have used these techniques to hide from a virus scanner. The viruses during this era used floppy disks and BBS's (bulletin board systems) to spread infected programs.
When Microsoft introduced Windows NT, the memory model was changed so that normal user programs could no longer modify key system tables. A lapse in hard virus technology followed, because no virus authors were using the new Windows kernel.
When the Internet began to catch on, it was dominated by UNIX operating systems. Most computers used variants of UNIX, and viruses were uncommon. However, this is also when network worms were born. With the famous Morris Worm, the computing world woke up to the possibility of software exploits.  During the early 1990s, many hackers figured out how to find and exploit buffer overflows, the "nuclear bomb" of all exploits. However, the virus-writing community didn't catch on for almost a decade.
During the early 1990s, a hacker would penetrate a system, set up camp, and then use the freshly compromised computer to launch new attacks. Once a hacker had penetrated a computer, she needed to maintain access. Thus, the first rootkits were born. These original rootkits were merely backdoor programs, and they used very little stealth. In some cases, they replaced key system binaries with modified versions that would hide files and processes. For example, consider a program called ls that lists files and directories. A first-generation rootkit might replace the ls program with a Trojan version that hides any file named hacker_stuff. Then, the hacker would simply store all of her suspect data in a file named hacker_stuff. The modified ls program would keep the data from being revealed.
System administrators at that time responded by writing programs such as Tripwire  that could detect whether files had been changed. Using our previous example, a security utility like Tripwire could examine the ls program and determine that it had been altered, and the Trojan would be unmasked.
The natural response was for attackers to move into the kernel of the computer. The first kernel rootkits were written for UNIX machines. Once they infected the kernel, they could subvert any security utility on the computer at that time. In other words, Trojan files were no longer needed: All stealth could be applied by modifying the kernel. This technique was no different from the techniques used by viruses in the late 1980s to hide from anti-virus software.