Threat Descriptons



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Trojan:OSX/DNSChanger are detections of installation packages, masked as fake codec installations for Mac OS X computers.


The F-Secure security product will automatically remove the file.

Restoring DNS Settings

  • Open the Apple Network Pane:
    • Applications >System Preferences >Network
  • Set the correct DNS Server, or leave it blank if you use a DHCP server.
  • To double check the DNS settings, click the Advanced button and choose the DNS tab:
  • Click the OK button, then the Apply button.

A False Positive is when a file is incorrectly detected as harmful, usually because its code or behavior resembles known harmful programs. A False Positive will usually be fixed in a subsequent database update without any action needed on your part. If you wish, you may also:

  • Check for the latest database updates

    First check if your F-Secure security program is using the latest updates, then try scanning the file again.

  • Submit a sample

    After checking, if you still believe the file is incorrectly detected, you can submit a sample of it for re-analysis.

    Note: If the file was moved to quarantine, you need to collect the file from quarantine before you can submit it.

  • Exclude a file from further scanning

    If you are certain that the file is safe and want to continue using it, you can exclude it from further scanning by the F-Secure security product.

    Note: You need administrative rights to change the settings.

Technical Details

These trojans start in the package install scripts.


Social engineering techniques are used to persuade the user into downloading and running this trojan. Websites hosting video (often illicit) claim that the video cannot be viewed without installing a new codec. The user is prompted to install the "needed" codec.

The user then downloads:

Installing the fake codec:

Once the fake codec is installed, the video will play so as not to raise suspicion. During the installation, the local machine's DNS settings are adjusted to point towards a malicious server.

Changes the DNS Server

The trojan changes the OS X network settings to use a different DNS server. DNS Settings are made with a tool called scutil.

The DNS Server Addresses vary. For example, Trojan:OSX/DNSChanger.A directs traffic to servers located in Ukraine.

Reports Back

After installation, the script sends back an HTTP message with information that it successfully infected the system. The message contains the operating system version and the host name.

Prevents Disinfection

The install script adds a crontab (a configuration file that specifies shell commands to run periodically on a given schedule) to a script to verify the malicious DNS servers remain unchanged. The script is stored in /Library/Internet Plug-Ins and is named plugins.settings.

The trojan infects both 10.4 and 10.5 versions of Mac OS X.

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