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Skulls.K

ALIAS:SymbOS/Skulls.K

Summary

Skulls.K is a variant of SymbOS/Skulls.C trojan, the main differences being that Skulls.K contains Cabir.M not Cabir.F and that Skulls.K contains similar flashing skull as Skulls.D

Skulls.K is a malicious SIS file trojan that replaces the system applications with non-functional versions, drops SymbOS/Cabir.M worm in to the phone and disables third party applications that could be used to disinfect with such as FExplorer, EFileman.

Skulls.K tries to disable F-Secure Mobile Anti-Virus by replacing it's files with non-functional versions. However as F-Secure Mobile Anti-Virus is capable of detecting Skulls.K using generic detection. The Anti-Virus will detect the infected SIS file and prevent it from being installed. Provided that the Anti-Virus is in realtime scan mode as it is by default.

The Cabir.M worm dropped by Skulls.K is already detected with generic detection as Cabir.Gen. So the Skulls.K is already detected and stopped without need for updated Anti-Virus database.

If Skulls.K is installed only the calling from the phone and answering calls works. All functions which need some system application, such as SMS and MMS messaging, web browsing and camera no longer function.

If you have installed Skulls.K, the most important thing is not to reboot the phone and follow the disinfection instruction in this description.

Disinfection

Disinfection with two Series 60 phones

Download F-Skulls tool from our FTP server to your computer or to a clean phone, then:

  • 1. Install F-Skulls.sis into infected phones memory card with a clean phone
  • 2. Put the memory card with F-Skulls into infected phone
  • 3. Start up the infected phone, the application menu should work now
  • 4. Go to application manager and uninstall the SIS file in which you installed the skulls variant
  • 5. Download and install F-Secure Mobile to remove any Cabirs dropped by the Skulls variant http://www.europe.f-secure.com/estore/avmobile.shtml or with mobile itself http://mobile.f-secure.com
  • 6. Remove the F-Skulls with application manager as the phone is now cleaned

Additional Details

Installation to system

Like Skulls.A Skulls.K is a SIS file that installs critical System ROM binaries and Cabir.M worm into C: drive. The System ROM files are installed with exact same names and locations as in the ROM drive.

Symbian operating system has a feature which causes any file that is in C: drive replace file in ROM drive with identical name and location.

Payload

Replaces built in applications with non-functional ones and installs Cabir.M worm.


Detection

Generic detection that detects Skulls.K was published for F-Secure Mobile Anti-Virus on December 13th, 2004 in database build number 15.

Write-up:Jarno Niemela May 9th, 2005;