1. Symantec/
  2. Security Response/
  3. Trojan.Dropper


Risk Level 1: Very Low

February 2, 2002
April 26, 2012 10:26:24 PM
Infection Length:
Systems Affected:
Trojan.Dropper is a detection name used by Symantec to identify malicious software programs that drop other malware files onto the compromised computer.

Background information

A Trojan.Dropper is a type of Trojan whose purpose is to deliver an enclosed payload onto a destination host computer. A dropper is a means to an end rather than the end itself. In other words, the dropper is usually used at the start or in the early stages of a malware attack.

Throughout the history of malicious software, the creators of malware have always looked for ways to bypass security software and stealthily install malware onto computers. Using a dropper is one method that is often used. Droppers primarily act as container files or an envelope in which to transport a malware payload from one computer to another. Once a dropper is executed, its own code is simply to load itself into memory and then extract the malware payload and write it to the file system. It may perform any installation procedures and execute the newly dropped malware. The dropper usually ceases to execute at this point as its primary function has been accomplished.

Malware authors use droppers as a method to confuse the user and make it more difficult for them to notice anything untoward that may be happening. For instance, after a user runs a file that is supposed to be a screen saver, they may see an error message. The message may mention something about a system error that caused the program to terminate. When the user closes the message box, he or she believes that the file has stopped execution, and technically they may be correct. What the user may not suspect, however, is that other files have been dropped and executed and are performing other malicious activities with the user being totally unaware.

Some droppers may also employ other more advanced techniques to help them bypass security programs such as encryption and packing of their payload.

Who creates droppers?
Trojan.Droppers are created by malware authors to help distribute their creations. They provide an easy, low cost, and low risk means of disguising their software and covering their tracks during malware distribution. For example, the droppers can be easily made to look like any other file or application by giving them different icons and file names to avoid them being recognized by users.

What is dropped?
The content that is dropped varies from one example to the next. The malicious files that are dropped may include executable file types such as .exe, .com, .scr and .dll. When these are executed they may perform any number of malicious actions.

Are there any tell-tale signs?
Droppers containing malicious software are generally discrete programs that attempt to mask their true purpose by masquerading as other software in the classic sense of the term Trojan horse. Some droppers may drop and execute their payloads while displaying a fake error message, while others may show nothing at all.

In the latter case it is possible that any tell-tale signs seen may be those caused by the dropped payload rather than the dropper itself. File system comparison programs run before and after execution of the dropper may show additional files created by the dropper if no rootkits are dropped and executed. Other signs may include registry changes or other system configuration changes that may be made by the dropper as it drops and installs its payload.

What are the risks?
The dangers posed by droppers are somewhat open-ended. As a dropper may enclose virtually anything at all, the scope of the damage it can potentially cause is only limited to what it can successfully dropped. On a positive note, even if the original dropper file itself is not detected by security software, there is a chance that the dropped payload may already be detected as an attempt is made to write it to the disk.

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.


Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

  • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
  • Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
  • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
  • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
  • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
  • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
  • If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
  • Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
  • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
  • Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
  • Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
  • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
  • For further information on the terms used in this document, please refer to the Security Response glossary.
Writeup By: Hon Lau
Summary| Technical Details| Removal

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