Concealment: Trojan horses, RootKits & Backdoors
Trojan horses
For a malicious program to accomplish its goals, it must be able to do so without being shut down, or deleted by the user or administrator of the computer it's running on. Concealment can also help get the malware installed in the first place. When a malicious program is disguised as something innocuous or desirable, users may be tempted to install it without knowing what it does. This is the technique of the Trojan horse or trojan.
Broadly speaking, a Trojan horse is any program that invites the user to run it, but conceals a harmful or malicious payload. The payload may take effect immediately and can lead to many undesirable effects, such as deleting all the user's files, or more commonly it may install further harmful software into the user's system to serve the creator's longer-term goals. Trojan horses known as droppers are used to start off a worm outbreak, by injecting the worm into users' local networks.
One of the most common ways that spyware is distributed is as a Trojan horse, bundled with a piece of desirable software that the user downloads from the Internet. When the user installs the software, the spyware is installed alongside. Spyware authors who attempt to act in a legal fashion may include an end-user license agreement which states the behavior of the spyware in loose terms, and which the users are unlikely to read or understand.
Root Kit Infection
Once a malicious program is installed on a system, it is often useful to the creator if it stays concealed. The same is true when a human attacker breaks into a computer directly. Techniques known as rootkits allow this concealment, by modifying the host operating system so that the malware is hidden from the user. Rootkits can prevent a malicious proces from being visible in the system's list of processes, or keep its files from being read. Originally, a rootkit was a set of tools installed by a human attacker on a Unix system where the attacker had gained administrator (root) access. Today, the term is used more generally for concealment routines in a malicious program.
Some malicious programs contain routines to defend against removal: not merely to hide themselves, but to repel attempts to remove them. An early example of this behavior is recorded in the Jargon File tale of a pair of programs infesting a Xerox CP-V timesharing system:
Each ghost-job would detect the fact that the other had been killed, and would start a new copy of the recently slain 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.
Similar techniques are used by some modern malware, wherein the malware starts a number of processes which monitor one another and restart any process which is killed off by the operator.
Backdoors
A Backdoor is a method of bypassing normal authentication procedures. Once a system has been compromised (by one of the above methods, or in some other way), one or more backdoors may be installed, in order to allow the attacker access in the future. The idea has often been suggested that computer manufacturers preinstall backdoors on their systems to provide technical support for customers, but this has never been reliably verified. Crackers typically use backdoors to secure remote access to a computer, while attempting to remain hidden from casual inspection. To install backdoors crackers may use Trojan horses, worms or other methods.
For a malicious program to accomplish its goals, it must be able to do so without being shut down, or deleted by the user or administrator of the computer it's running on. Concealment can also help get the malware installed in the first place. When a malicious program is disguised as something innocuous or desirable, users may be tempted to install it without knowing what it does. This is the technique of the Trojan horse or trojan.
Broadly speaking, a Trojan horse is any program that invites the user to run it, but conceals a harmful or malicious payload. The payload may take effect immediately and can lead to many undesirable effects, such as deleting all the user's files, or more commonly it may install further harmful software into the user's system to serve the creator's longer-term goals. Trojan horses known as droppers are used to start off a worm outbreak, by injecting the worm into users' local networks.
One of the most common ways that spyware is distributed is as a Trojan horse, bundled with a piece of desirable software that the user downloads from the Internet. When the user installs the software, the spyware is installed alongside. Spyware authors who attempt to act in a legal fashion may include an end-user license agreement which states the behavior of the spyware in loose terms, and which the users are unlikely to read or understand.
Root Kit Infection
Once a malicious program is installed on a system, it is often useful to the creator if it stays concealed. The same is true when a human attacker breaks into a computer directly. Techniques known as rootkits allow this concealment, by modifying the host operating system so that the malware is hidden from the user. Rootkits can prevent a malicious proces from being visible in the system's list of processes, or keep its files from being read. Originally, a rootkit was a set of tools installed by a human attacker on a Unix system where the attacker had gained administrator (root) access. Today, the term is used more generally for concealment routines in a malicious program.
Some malicious programs contain routines to defend against removal: not merely to hide themselves, but to repel attempts to remove them. An early example of this behavior is recorded in the Jargon File tale of a pair of programs infesting a Xerox CP-V timesharing system:
Each ghost-job would detect the fact that the other had been killed, and would start a new copy of the recently slain 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.
Similar techniques are used by some modern malware, wherein the malware starts a number of processes which monitor one another and restart any process which is killed off by the operator.
Backdoors
A Backdoor is a method of bypassing normal authentication procedures. Once a system has been compromised (by one of the above methods, or in some other way), one or more backdoors may be installed, in order to allow the attacker access in the future. The idea has often been suggested that computer manufacturers preinstall backdoors on their systems to provide technical support for customers, but this has never been reliably verified. Crackers typically use backdoors to secure remote access to a computer, while attempting to remain hidden from casual inspection. To install backdoors crackers may use Trojan horses, worms or other methods.