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


Risk Level 1: Very Low

September 12, 2002
May 21, 2010 2:57:13 AM
Systems Affected:
Trojan.Adclicker is a detection name used by Symantec to identify malicious programs that share the primary functionality of artificially generating traffic to pay-per-click Web advertising campaigns in order to create or boost revenue.

Background information

When making use of Web-based advertising campaigns, advertisers typically pay advertisement publishers each time a user clicks on one of their advertisements. A typical sequence of events is:

  1. An advertiser creates an advertisement and sends it to a publisher
  2. A publisher publishes the advertisement so that it is displayed to users
  3. Users click on the advertisement and are redirected to the advertiser's website
  4. The advertiser pays a fee to the publisher for each of the users' clicks

If a criminal group or their affiliates pose as a publisher, they can generate revenue from the advertiser(s) by artificially generating fake 'clicks'. One way in which this may be achieved is through the use of a Trojan horse program that runs on unsuspecting users' computers.

Advertising networks
The above sequence is complicated by the existence of Web advertising networks that act as a middleman between the advertiser and the publisher; these days, most Web advertising campaigns are conducted with the aid of such a network. The networks typically allow advertisers to select and bid for certain keywords to be associated with their advertisements, with the advertising networks being able to display appropriate advertisements to users, often based on their browsing habits. The above sequence of events in this case should be:

  1. An advertiser creates an advertisement, selects certain keywords, and sends them to an advertising network
  2. The advertising network places the advertisement on certain publishers' websites and on pages that are likely to be of interest to the user
  3. Users click on the advertisement and are redirected to the advertiser's website
  4. The advertiser pays a fee to the advertising network for each of the users' clicks
  5. The advertising network pays a portion of those fees to the publisher

The same Trojan may be used whether or not an advertising network is in place; assuming that the pay-per-click fraud goes undetected, the advertising network must still pay the 'publisher', which in this case is the criminal group. The fraudsters therefore still profit from the scam.

Note that in some cases, the advertising network and the publisher may be the same entity.

How does the Trojan 'click' on the advertisements?
The primary functionality of this family of Trojans is to generate fake 'clicks' on Web advertisements by sending HTTP GET requests to predetermined URLs. These requests are the same as those that may be sent by a Web browser when a legitimate user is browsing the Web. The Trojan may also falsify the 'referrer' field in the HTTP request in order to mask the fake nature of the access.

Other functionality
Some programs detected as Trojan.Adclicker may be designed to perform other actions. Criminals may produce programs to 'click' on their competitors' advertisements, thus costing them money. Alternatively, personal or political factors may be involved. The programs may also be used to generate revenue though referrer fees by creating fake 'clicks' on certain websites or advertising campaigns.

What happens after the Trojan is installed?
Programs detected as Trojan.Adclicker typically copy themselves to a system folder in an attempt to remain inconspicuous, and create a load point so that they run every time Windows starts. The Trojans may also perform the following actions:
  • Lower security settings
  • Attempt to download files, including other malware
  • Display messages and/or advertisements

The Trojans typically then begin their routines to generate fake clicks.

Are there any tell tale signs?
In general, programs detected as Trojan.Adclicker aim to execute without the knowledge of the user, but indicators of infection may include slow or jittery Internet browsing. In some cases the Trojans may consume significant bandwidth. Some variants of Trojan.Adclicker may additionally display messages and/or advertisements on the compromised computer.

What are the risks?
The most immediate risk to the user is that of Internet use being disrupted. This may be a direct result of the bandwidth being consumed by the threats, or in extreme cases users may find that their ISP throttles their connection or disconnects it altogether. Alternatively, users may find that they are charged extra by their ISP once they have consumed more than an allotted monthly bandwidth cap.

Users may be vulnerable to the effects of other malware that may be downloaded by the threats.

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) -- may help to block back channel activities initiated by these types of malicious programs. Program controls such as those found in Symantec Endpoint Protection can also help to prevent unknown 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: Henry Bell
Summary| Technical Details| Removal

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