Friday, December 21, 2007

Trojan horse

A Trojan horse, or simply Trojan, is a piece of software which appears to perform a certain action but in fact performs another. Contrary to popular belief, this action, usually encoded in a hidden payload, may or may not be acutely malicious, but Trojan horses are notorious today for their use in the installation of backdoor programs. Simply put, a Trojan horse is not a computer virus. Unlike such badware, it does not propagate by self-replication but relies heavily on the exploitation of an end-user (see Social engineering). It is instead a categorical attribute which can encompass many different forms of codes. Therefore, a computer worm or virus may be a Trojan horse. The term is derived from the classical myth of the Trojan horse.

In the field of computer architecture, 'Trojan Horse' can also refer to security loopholes that allow kernel code to access anything for which it is not authorized.

Trojan horse payloads are almost always designed to do various harmful things, but can also be harmless. They are broken down in classification based on how they breach and damage systems.

The nine main types of Trojan horse payloads are:

Remote Access

Email Sending

Data Destruction


Proxy Trojan (disguising others as the infected computer)

FTP Trojan (adding or copying data from the infected computer)

Security software disabler

Denial-of-service attack (DoS)

URL trojan (directing the infected computer to only connect to the internet via an expensive dial-up connection)

Some examples of damage are:

erasing or overwriting data on a computer

encrypting files in a cryptoviral extortion attack

corrupting files in a subtle way

upload and download files

allowing remote access to the victim's computer. This is called a RAT (remote administration tool)

spreading other malware, such as viruses: this type of Trojan horse is called a 'dropper' or 'vector'

setting up networks of zombie computers in order to launch DDoS attacks or send spam.

spying on the user of a computer and covertly reporting data like browsing habits to other people

making screenshots

logging keystrokes to steal information such as passwords and credit card numbers

phishing for bank or other account details, which can be used for criminal activities

installing a backdoor on a computer system

opening and closing CD-ROM tray

harvesting e-mail addresses and using them for spam

restarting the computer whenever the infected program is started

deactivating or interfering with anti-virus and firewall programs

deactivating or interfering with other competing forms of malware

randomly shutting off your computer

Methods of infection

The majority of Trojan horse infections occur because the user was tricked into running an infected program. This is why it is advised not to open unexpected attachments on emails -- the program is often a cute animation or an image, but behind the scenes it infects the computer with a Trojan or worm. The infected program doesn't have to arrive via email; it can be sent in an Instant Message, downloaded from a Web site or by FTP, or even delivered on a CD or floppy disk. (Physical delivery is uncommon, but if one were the specific target of an attack, it would be a fairly reliable way to infect a computer.) Furthermore, an infected program could come from someone who sits down at a computer and loads it manually. However, receiving a Trojan in this manner is very rare. It is usually received through a download.

How Trojan Horses Are Installed

Users can be tricked into installing Trojan horses by being enticed or frightened. For example, a Trojan horse might arrive in email described as a computer game. When the user receives the mail, they may be enticed by the description of the game to install it. Although it may in fact be a game, it may also be taking other action that is not readily apparent to the user, such as deleting files or mailing sensitive information to the attacker. As another example, an intruder may forge an advisory from a security organization, such as the CERT Coordination Center, that instructs system administrators to obtain and install a patch.

Other forms of "social engineering" can be used to trick users into installing or running Trojan horses. For example, an intruder might telephone a system administrator and pose as a legitimate user of the system who needs assistance of some kind. The system administrator might then be tricked into running a program of the intruder's design.

Software distribution sites can be compromised by intruders who replace legitimate versions of software with Trojan horse versions. If the distribution site is a central distribution site whose contents are mirrored by other distribution sites, the Trojan horse may be downloaded by many sites and spread quickly throughout the Internet community.

Because the Domain Name System (DNS) does not provide strong authentication, users may be tricked into connecting to sites different than the ones they intend to connect to. This could be exploited by an intruder to cause users to download a Trojan horse, or to cause users to expose confidential information.

Intruders may install Trojan horse versions of system utilities after they have compromised a system. Often, collections of Trojan horses are distributed in toolkits that an intruder can use to compromise a system and conceal their activity after the compromise, e.g., a toolkit might include a Trojan horse version of ls which does not list files owned by the intruder. Once an intruder has gained administrative access to your systems, it is very difficult to establish trust in it again without rebuilding the system from known-good software

Finally, a Trojan horse may simply be placed on a web site to which the intruder entices victims. The Trojan horse may be in the form of a Java applet, JavaScript, ActiveX control, or other form of executable content.


The best advice with respect to Trojan horses is to avoid them in the first place.

System administrators (including the users of single-user systems) should take care to verify that every piece of software that is installed is from a trusted source and has not been modified in transit. When digital signatures are provided, users are encouraged to validate the signature (as well as validating the public key of the signer). When digital signatures are not available, you may wish to acquire software on tangible media such as CDs, which bear the manufacturer's logo. Of course, this is not foolproof either. Without a way to authenticate software, you may not be able to tell if a given piece of software is legitimate, regardless of the distribution media.

I strongly encourage software developers and software distributors to use cryptographically strong validation for all software they produce or distribute. Any popular technique based on algorithms that are widely believed to be strong will provide users a strong tool to defeat Trojan horses.

Anyone who invests trust in digital signatures must also take care to validate any public keys that may be associated with the signature. It is not enough for code merely to be signed -- it must be signed by a trusted source.

Do not execute anything sent to you via unsolicited electronic mail.

Use caution when executing content such as Java applets, JavaScript, or Active X controls from web pages. You may wish to configure your browser to disable the automatic execution of web page content.

Apply the principle of least privilege in daily activity: do not retain or employ privileges that are not needed to accomplish a given task. For example, do not run with enhanced privilege, such as "root" or "administrator," ordinary tasks such as reading email.

Install and configure a tool such as Tripwire® that will allow you to detect changes to system files in a cryptographically strong way

Educate your users regarding the danger of Trojan horses.

Use firewalls and virus products that are aware of popular Trojan horses. Although it is impossible to detect all possible Trojan horses using a firewall or virus product (because a Trojan horse can be arbitrary code), they may aid you in preventing many popular Trojan horses from affecting your systems.

Review the source code to any open source products you choose to install. Open source software has an advantage compared to proprietary software because the source code can be widely reviewed and any obvious Trojan horses will probably be discovered very quickly. However, open source software also tends to be developed by a wide variety of people with little or no central control. This makes it difficult to establish trust in a single entity. Keep in mind that reviewing source code may be impractical at best.

Adopt the use of cryptographically strong mutual authentication systems, such as ssh, for terminal emulation, X.509 public key certificates in web servers, S/MIME or PGP for electronic mail, and kerberos for a variety of services. Avoid the use of systems that trust the domain name system for authentication, such as telnet, ordinary http (as opposed to https), ftp, or smtp, unless your network is specifically designed to support that trust.

Do not rely on timestamps, file sizes, or other file attributes when trying to determine if a file contains a Trojan horse.

Exercise caution when downloading unauthenticated software. If you choose to install software that has not been signed by a trusted source, you may wish to wait for a period of time before installing it in order to see if a Trojan horse is discovered.

We encourage all security organizations to digitally sign any advisories or other alerts. We also recommend that users validate any signatures, and beware of unsigned security advice.

If you do fall victim to a Trojan horse, some anti-virus software may also be able to recognize, remove and repair the damage from the Trojan horse. However, if an intruder gains access to your systems via a Trojan horse, it may be difficult or impossible to establish trust in your systems. In this case, we recommend that you disconnect from the network and rebuild your systems from known-good software, being careful to apply all relevant patches and updates, to change all passwords, and to check other nearby systems


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