"The intruder could use a rootkit to hide the password cracker
program that’s stealing your passwords and sending them back
to the intruder. The intruder could also use a rootkit to hide a
“back door” program that would give him easy access
back into the compromised system. There are at least six basic
categories of rootkits which all serve the same purpose. That is,
they prevent the intruder’s malicious software from showing
screen output to the unsuspecting user, and they prevent the
malicious software from leaving traces in the system logs. They
also prevent the malicious software from showing up in a
“ps” or “top” process list.
"One of the most difficult rootkits to discover is the firmware
rootkit that is placed in the code that exists in the ACPI or PCI
cards or your system clock. Firmware rootkits can be installed in
any flashable code on your motherboard or any cards that you
install. The difficulties here will be that you cannot fix this by
reinstalling your operating system or wiping your hard drives."
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