Tools
Trojan-Downloader:OSX/Flashback.B
Summary
Disinfection
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 Instructions
- 1. Run the following command in Terminal:
defaults read /Applications/Safari.app/Contents/Info LSEnvironment
- 2. Take note of the value, DYLD_INSERT_LIBRARIES.
- 3. Run the following command in Terminal:
defaults read /Applications/Firefox.app/Contents/Info 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/Safari.app/Contents/Info LSEnvironment
sudo chmod 644 /Applications/Safari.app/Contents/Info.plist
sudo touch /Applications/Safari.app
sudo defaults delete /Applications/Firefox.app/Contents/Info LSEnvironment
sudo chmod 644 /Applications/Firefox.app/Contents/Info.plist
sudo touch /Applications/Firefox.app
- 6. Delete the files obtained in steps 2 and 4.
Protection
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Additional 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.
Payload
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/Safari.app/Contents/Resources/%payload_filename%
- /Applications/Firefox.app/Contents/Resources/%payload_filename%
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.
Example:
- The following line is inserted into "/Applications/Safari.app/Contents/Info.plist":
- <key>LSEnvironment</key><dict><key>DYLD_INSERT_LIBRARIES</key>
<string>/Applications/Safari.app/Contents/Resources/%payload_filename%</string></dict>
- <key>LSEnvironment</key><dict><key>DYLD_INSERT_LIBRARIES</key>
- The following line is inserted to "/Applications/Firefox.app/Contents/Info.plist":
- <key>LSEnvironment</key><dict><key>DYLD_INSERT_LIBRARIES</key>
<string>/Applications/Firefox.app/Contents/Resources/%payload_filename%</string></dict>
- <key>LSEnvironment</key><dict><key>DYLD_INSERT_LIBRARIES</key>
The installer then restarts running instances of Safari and Firefox, to take the payload into effect.