Trojan Horse

Trojan Horse is a detection name used by Symantec to identify malicious software programs that masquerade as benign applications or files.


Background information
Trojan horse programs are named after one of the most famous deceptions in history. In ancient Troy, the Greek army appeared to leave the war ravaged shores of Troy, but left a large wooden horse in what appeared to be a peace gift. Within the Trojan horse lay an elite group of Greek warriors, unbeknownst to the City of Troy. Once inside the City, the elite warriors quickly and efficiently performed their planned operation and captured the City.

Software Trojans masquerade as an application or file that entice a user to open it. A Trojan horse may copy itself on to the compromised computer, but it doesn't make copies of itself and spread like a virus which is a key difference between a Trojan and a virus. While most Trojans only execute their own malicious code, some Trojans may actually perform the actions of the file they pretend to be, but then they execute their own malicious code on the compromised computer. Other Trojans make it appear that they are performing the desired actions, but in reality do nothing but trigger their malicious routines.

Trojans arrive on to compromised computers in a variety of ways. These methods distribute the Trojan, often as rapidly as possible, so that the Trojan can maximize the opportunity to perform its main function in a large user population before they are detected by antivirus software.

One of the most common methods is for the Trojan to be spammed as an email attachment or a link in an email. Another similar method has the Trojan arriving as a file or link in an instant messaging client. These methods often rely on social engineering techniques to tempt the user to click on the link or open the file since many of these emails and instant messages appear to come from people the user knows. These techniques will play on a user's curiosity about the big new item such as a celebrity scandal, crisis, catastrophe, or major global event.

Another means of arrival includes a method called drive-by downloads. A drive-by download occurs when a user goes to a web site that is either legitimate, but compromised and exploited or malicious by design. The download occurs surreptitiously without the user's knowledge. Alternatively, the user is asked to update or add a video codec when at a malicious web site. When the user complies with this request, they inadvertently download a Trojan pretending to be the video codec.

Finally, a Trojan horse program can be dropped or downloaded by other malicious software or by legitimate programs that have been compromised or exploited on the compromised computer.

Just as each Greek warrior had his own task to perform in capturing Troy, there are several types of Trojans, each with particular functions. Some Trojans perform multiple functions and have the prefix of Trojan, while others are categorized by their main functions.

• Backdoor Trojans are stealthy and allows remote access to the compromised computer by opening a back door.
• Downloader Trojans are aptly named as they download additional files on to the compromised computer which may be additional malware or updates of the Trojan.
• Infostealing Trojans gather confidential information from the computer and sends it to a predetermined location. This information can be financial, related to the compromised computer or user credentials for various web sites.

Who creates Trojans?
Trojan horse programs were once created by malware authors for an assortment of reasons, most especially the infamy of destruction and damage and to make a name for themselves by proving they could write malicious programs. Trojans are now generally created by malware authors with the intent of making a profit.


What happens after the Trojan is installed?
Once it is executed on the compromised computer, a Trojan horse program may create files and registry entries. It may copy itself to various locations. It may start a service or inject itself into processes and then carry out its primary functions.


What can Trojans do?
Trojans can perform a large variety of actions. Some Trojan actions that are most commonly seen include:

• Distributed Denial of Service
• Downloading files
• Dropping additional malware
• Disabling security-related programs
• Opening a back door
• Stealing confidential and financial information

Are there any tell-tale signs?
As deception is one of the hallmarks of Trojan horse programs, many will run with as much stealth as possible. This means that, in the majority of cases, there will not be any obvious tell-tale signs that they are running on a computer. There are some Trojans that may display messages or dialog boxes and some that may display picture files or open a text file.


What are the risks?
Damage from Trojans range from a relatively minimal risk of annoyance and nuisance to a high risk of destruction or loss to the user. Hidden files, modified registry entries and annoying but harmless displays of pictures or error messages are examples of some of the low risk actions associated with Trojans.

On the other end of the scale, the potential for identity theft is high and is a risk considered to be personally damaging to a user. Another high and potentially destructive risk is the opening of a back door that can allow a remote attacker access to the compromised computer to perform many actions, such as:

• Create administrator accounts
• Participate in a Distributed Denial of Service (DDoS)
• Provide confidential computer information
• Redirect GRE, TCP, HTTP, HTTPS, SOCKS4 and SOCKS5 traffic

What can I do to minimize the risks?
As a general rule, users should always run up-to-date antivirus software with real-time protection such as Norton Antivirus, Norton Internet Security, Norton 360 or Symantec Endpoint Protection. In addition, a firewall -- or better still, an Intrusion Prevention System (IPS) -- will help to block download activities initiated by these types of malicious programs. Program controls such as those found in Symantec Endpoint Protection can also help to prevent programs such as these from executing in the first place.


How can I find out more?
Advanced users can submit a sample to Threat Expert to obtain a detailed report of the system and file system changes caused by a threat.