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.
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 detection database 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.
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.
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.
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.
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.