Trojan-Downloader:OSX/Flashback.K

Trojan-Downloader:OSX/Flashback.K is dropped by malicious Java applets that exploit the known CVE-2012-0507 vulnerability.

On execution, the malware will prompt the unsuspecting user for the administrator password. Whether or not the user inputs the administrator password, the malware will attempt to infect the system, though entering the password will affect how the infection is done.

Installation

There are two files that are dropped and executed on the system when users visited a malicious webpage.

The first file is an updater component. It is dropped in the users home folder. It may have the default filename ".jupdate" or a filename supplied by the malicious webpage. The filename will always start with a ".".

A launch point is then created for the updater component in the ~/Library/LaunchAgents folder. It may have the default filename "com.java.update.plist" or a filename supplied by the malicious webpage.

On the first execution, this component reports to the following:

• http://[...]31.31.79.87/[...]/stat_svc/

On the second execution and onwards, it connects to a hard coded list of addresses to download it's update.

The second file is the downloader component just like the previous variants. It is dropped and executed in the /tmp folder. It may have the default filename "Update" or a filename supplied by the malicious webpage.

The malware then reports to the following location whether it successfully exploited the system or not:

• http://[...]31.31.79.87/[...]/stat_j/%result%

Downloading the Payload

The malware connects to the following URL to download its payload:

• http://[...]31.31.79.87/[...]/counter/%encoded_data% Where decoded data follows this format:
  • %hardware_UUID|%machine_architecture%|%kernel_version%|0|%architecture_of_malware_process%| %current_hardware_type_of_system%|%is_user_daemon%
    • %is_user_daemon% is "1" if the process is running as the first OS X user account or daemon "0" otherwise

The filename and actual content of the payload depends on reply of the remote host. The reply is compressed and encrypted but the actual content follows this format:

• %encoded_filename%|%encoded_binary1_content%|%encoded_payload_config%| %encoded_binary2_content%|%encoded_png_content% Where:
  • Binary 1: We were not able to obtain the payload during our analysis. However based on previous variants, binary1 is most likely the malware's main component. It hijacks CFReadStreamRead and CFWriteStreamWrite by creating an interposition to these functions. The malware modifies contents returned or send by these APIs. It targets the contents of specific webpages, as determined by config information returned by the remote host.
  • Binary 2: We were not able to obtain the payload during our analysis. However based on previous variants, binary2 is most likely a filter component that will load binary1 only into a targeted process. This is to avoid crashing incompatible applications and raising the user's suspicions. In the sample that we analyzed, it targets the Safari web browser.

Infection

Only after downloading the payload does Flashback.K proceed with infecting the machine. To do so, the malware prompts for the administrator password, as in the following screenshot:

The icon indicated by the red box in the screenshot is the PNG content returned by the remote host. This is dropped to the location '/tmp/.i.png' on the system. Since this image is controlled by the remote host, it can be changed any time the author deems necessary.

Whether or not the user inputs their administrator password at the prompt determines the type of infection the malware subsequently performs:

Infection Type 1

If the user inputs their administrator password, the malware will create the following files:

• /Applications/Safari.app/Contents/Resources/.%decoded_filename%.png - contains %decoded_binary1_contents% and %decoded_payload_config%
• /Applications/Safari.app/Contents/Resources/.%decoded_filename%.xsl - contains %decoded_binary2_contents%

The malware then creates a launch point, inserting the following line into "/Applications/Safari.app/Contents/Info.plist":

• <key>LSEnvironment</key><dict><key>DYLD_INSERT_LIBRARIES</key> <string>/Applications/Safari.app/Contents/Resources/.%decoded_filename%.xsl</string></dict>

This in effect will inject binary2 into Safari when the browser is launched.

If the malware was able to infect the system this way, it reports success to the following URL:

• http://[...]31.31.79.87/[...]/stat_d/

If it failed to infect the system, the malware reports to the following URL:

• http://[...]31.31.79.87/[...]/stat_n/

Infection Type 2

In cases where the user did not input their administrator password, the malware checks if the following path exists in the system:

• /Applications/Microsoft Word.app
• /Applications/Microsoft Office 2008
• /Applications/Microsoft Office 2011
• /Applications/Skype.app

If any of these are found, the malware again skips the rest of its routine and proceeds to delete itself, presumably to avoid infecting a system that has an incompatible application installed.

If none of the incompatible applications are found, the malware will create the following files:

• ~/Library/Application Support/.%decoded_filename%.tmp - contains %decoded_binary1_contents% and %decoded_payload_config%
• /Users/Shared/.libgmalloc.dylib - contains %decoded_binary2_contents%

The malware then creates a launch point by creating "~/.MacOSX/environment.plist", containing the following lines:

• <key>DYLD_INSERT_LIBRARIES</key> <string>/Users/Shared/.libgmalloc.dylib</string>

This in effect will inject binary2 into every application launched by the infected user.

For this infection type, the malware reports the successful infection to the following URL:

• http://[...]31.31.79.87/[...]/stat_u/