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


Risk Level 1: Very Low

April 4, 2002
April 26, 2010 2:46:01 PM
Infection Length:
Systems Affected:
Downloader.Trojan is a detection name used by Symantec to identify malicious software programs that share the primary functionality of downloading content.

Background information

Traditionally, software distribution involved making software installers available to users through physical media such as CDs or DVDs. This meant that full installers containing all the files required for a software application could be provided as long as there was enough space on the media.

With the advent of the Internet, more and more software began to be distributed through the network. Due to bandwidth limitations and the increased frequency of software updates it became more economical to distribute so-called stub installers in place of full installers, thus reducing the amount of content that needed to be downloaded. Instead of downloading several hundred megabytes before installing and customizing the components of the software, users download a much smaller file, in the order of hundreds of kilobytes. After the stub installer is run and the user makes the required choices, the stub installer determines an optimized list of components that are required. This is in contrast to downloading all components of a package, many of which would not be needed. For these reasons, stub installer programs are widely used by many legitimate software applications.

Malware downloaders operate in a similar fashion and copy techniques from the mainstream software industry. Malware downloaders offer an attractive level of indirection and economy to would-be attackers. Stub downloader files are more likely to slip through unnoticed by a user than if a much larger installer package were used, which may take some time to download. Furthermore, when an attack is split into a chain of smaller components it spreads the risk of failure.

If, following action by antivirus software producers, the downloader components of an attack sequence are detected by antivirus software, the attackers can easily create new downloader components and reuse the other parts of the attack. Furthermore, the risk of multiple separate attack components being detected by security software is much lower than if a single file were used. This strategy is one of risk distribution and is the exact opposite of putting "all your eggs in one basket".

This pattern of behavior -- using distributed code components -- reflects the move by mainstream software providers to shift their software and services into the network world otherwise known as the "cloud", where functionality and software components are downloaded and accessed on-demand rather than permanently installed on the local computer.

Who creates downloaders?
Downloaders are created by the whole spectrum of malware authors. They provide an easy, low cost, low risk and reusable means of malware distribution. This means that they are commonly used in many malware-based attacks.

What is downloaded?
The content that is downloaded varies from one example to the next. It may comprise of, but need not be limited to, the following items:
  • Configuration/command information
  • Miscellaneous files
  • Other threats or security risks such as components related to pay per install operations
  • Misleading Applications
  • Secondary components of, or upgrades to, the existing attack

Where is it downloaded from?

Most downloaders that are encountered will attempt to download content from the Internet rather than the local network. Downloaders will usually have URLs directly embedded into their code in some shape or form, meaning that the download location is fixed. In contrast, some downloaders may contain the functionality to download content but must be given a parameter or command to specify the download location. These are likely to be dropped by other threats and then used to download other files.

Are there any tell-tale signs?
Downloaders associated with malicious software are generally small and discrete programs that attempt to carry out their functionality whilst staying below the radar. This means that, in the majority of cases, there will not be any obvious tell-tale signs that they are running on a computer. In some cases a downloader may be suspected if network performance is significantly affected, with the available bandwidth being used by the threat. Alternatively, a user may find unfamiliar files on the file system where they may not have been present before. It should be noted, however, that these are by no means definitive indicators of the presence of a downloader.

What are the risks?
The dangers posed by downloaders are open-ended but are usually time-bound. As a downloader may download virtually anything at all, the scope of the damage it can potentially cause is only limited to what it can successfully download. On a positive note, malicious sites associated with downloaders often have a limited life span before being closed down or cleaned up. This means that older downloaders may present a minimal risk to users even when they are not detected by security software.

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.
Summary| Technical Details| Removal

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