Rookits

Once a malicious program is installed on a system, it is essential that it stay concealed, to avoid detection. Techniques known as rootkits allow this concealment, by modifying the host's operating system so that the malware is hidden from the user. Rootkits can prevent a malicious process from being visible in the system's list of processes, or keep its files from being read.

Some malicious programs contain routines to defend against removal, not merely to hide themselves, but to resist attempts to remove them. An early example of this behavior is recorded in theJargon File tale of a pair of programs infesting a Xerox CP-V time sharing system:

Each ghost-job would detect the fact that the other had been killed, and would start a new copy of the recently stopped program within a few milliseconds. The only way to kill both ghosts was to kill them simultaneously (very difficult) or to deliberately crash the system.