Threat Explorer

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09 April 2012
13 April 2012
Also Known As:
Trojan-Downloader.OSX.Flashfake.ab [Kaspersky]
Infection Length:
Systems Affected:
CVE References:
OSX.Flashback.K is a Trojan horse that opens a back door on the compromised computer.

Note: Virus definitions dated April 9, 2012 or earlier detect this threat as OSX.Flashback .

For more information, please see the following resources:

Antivirus Protection Dates

  • Initial Rapid Release version 09 April 2012 revision 035
  • Latest Rapid Release version 26 April 2019 revision 003
  • Initial Daily Certified version 10 April 2012 revision 003
  • Latest Daily Certified version 26 April 2019 revision 006
  • Initial Weekly Certified release date 11 April 2012
Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.
This Trojan may arrive on the compromised computer by exploiting the Oracle Java SE Remote Java Runtime Environment Denial Of Service Vulnerability (BID 52161).

When executed, the Trojan creates a semaphore with the following name:

Next, it attempts to run the following file using the system() command:

The Trojan then obtains the following system information from the compromised computer:
  • HW.machine
  • Kernel information
  • Machine type
  • OS information
  • Password
  • User ID

The Trojan deletes itself if any of the following applications are present on the computer:
  • /Library/Little Snitch
  • /Developer/Applications/
  • /Applications/VirusBarrier
  • /Applications/iAntiVirus/
  • /Applications/avast!.app
  • /Applications/
  • /Applications/
  • /Applications/Packet

If none of the above applications are present, the Trojan connects to the following location and downloads more files on to the compromised computer:

The downloaded files are compressed and encrypted using RC4. It then decrypts and decompresses the files.

The Trojan may then modify one of the following files:
  • /Applications/
  • ~/.MacOSX/environment.plist

It then sets the binary to the environment variable "DYLD_INSERT_LIBRARIES" using the command "launchctrl setenv DYLD_INSERT_LIBRARIES=" and exports it to ensure that it runs when the compromised computer starts.

Next, it connects to one of the following remote locations depending on successful installation:
  • [http://][REMOVED]
  • [http://][REMOVED]
  • [http://][REMOVED]


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.
Symantec has made available a removal tool:
Download a copy of the OSX.Flaskback.K removal tool .

The following additional instructions pertain to all current and recent Symantec antivirus products for Macintosh.
  1. Update the virus definitions.
  2. Run a full system scan and repair or delete all the files detected.

For specific details on each of these steps, read the following instructions.

1. To update the virus definitions
Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions:
  • Running LiveUpdate, which is the easiest way to obtain virus definitions: These virus definitions are posted to the LiveUpdate servers once each week (usually on Wednesdays), unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, refer to the Virus Definitions (LiveUpdate).
  • Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). You should download the definitions from the Symantec Security Response website and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the Virus Definitions (Intelligent Updater).

The Intelligent Updater virus definitions are available: Read "How to update virus definition files using the Intelligent Updater " for detailed instructions.

2. To scan for and delete the infected files
  • Start your Norton AntiVirus or Symantec Endpoint Protection for Macintosh program and make sure that it is configured to scan all files.
  • Run a full system scan.
  • If any files are detected, click Repair (if available) or Delete.
Writeup By: Karthik Selvaraj