Trojan-Downloader:OSX/Flashback.B is a variant of the Trojan-Downloader:OSX/Flashback malware that poses as a Flash Player installer, and connects to a remote host to obtain installation configurations and files.

Free Removal Tool

11 April 2012: F-Secure now provides a free removal tool that automates the detection and removal of Flashback variants from an infected machine.

Further information and download of the tool is available in the following Labs Weblog post:

Manual removal

Caution: Manual disinfection is a risky process; it is recommended only for advanced users. Otherwise, please seek professional technical assistance.

F-Secure customers may also contact our Support.

Manual removal

  1. Run the following command in Terminal:
    • defaults read /Applications/ LSEnvironment
  2. Take note of the value, DYLD_INSERT_LIBRARIES.
  3. Run the following command in Terminal:
    • defaults read /Applications/ LSEnvironment
  4. Take note of the value, DYLD_INSERT_LIBRARIES.
  5. Run the following commands in Terminal (first make sure there is only one entry, from steps 2 and 4):
    • sudo defaults delete /Applications/ LSEnvironment
    • sudo chmod 644 /Applications/
    • sudo touch /Applications/
    • sudo defaults delete /Applications/ LSEnvironment
    • sudo chmod 644 /Applications/
    • sudo touch /Applications/
  6. Delete the files obtained in steps 2 and 4.

Suspect a file is incorrectly detected (a False Positive)?

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.

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Technical Details

Trojan-Downloader:OSX/Flashback.B poses as a Flash Player installer and connects to a remote host to obtain further installation files and configurations.

Screenshot of the Trojan-Downloader:OSX/Flashback.B installer.

Unlike the first variant, the user is required to key in the administrator password to complete the installation / infection.

On installation, the installer first checks if the following file is found in the system:

  • /Library/Little Snitch/lsd

Little Snitch is a firewall program for Mac OS X. If the program is found, the installer will skip the rest of its routine and proceed to delete itself.

If the trojan is cleared to proceed, it connects to a remote host, identified as h t t p : / / macosxsoftwareupdate .org/counter/%encoded_strings%, with the decoded string following this format:

  • %Hardware_UUID% | %machine_architecture% | %kerner_version%

As of this writing, the remote host is not accessible.


Installation files and configuration returned by the host is encrypted using RC4, where the MD5 hash of the Hardware UUID of the infected system is used as the key. The decrypted content follows this format:

  • %encoded_payload_filename% | %encoded_payload_content%

The installer drop copies of the payload to the following locations:

  • /Applications/
  • /Applications/

A DYLD_INSERT_LIBRARIES environment variable is also added to the targeted browsers as launch point. This is done by inserting a LSEnvironment entry to the corresponding Info.plist of the browsers.


  • The following line is inserted into "/Applications/":
    • <key>LSEnvironment</key><dict><key>DYLD_INSERT_LIBRARIES</key><string>/Applications/</string></dict>
  • The following line is inserted to "/Applications/":
    • <key>LSEnvironment</key><dict><key>DYLD_INSERT_LIBRARIES</key><string>/Applications/</string></dict>

The installer then restarts running instances of Safari and Firefox, to take the payload into effect.