Threat Description

Trojan-Dropper:​W32/Blocker.A

Details

Aliases:Trojan-Dropper:​W32/Blocker.A, Trojan.Dropper.Blocker.A
Category:Malware
Type:Trojan-Dropper
Platform:W32

Summary



This type of trojan contains one or more malicious programs, which it will secretly install and execute.



Removal



Rescue CD

Use the F-Secure Rescue CD with an updated database to scan the Windows Partition. Then re-scan the system after reboot to delete the file renamed by Rescue CD. Reset the default value of the registry "Userinit" or remove the added entry.

Note

Removing Blocker.A may require a third party tool to access the directory where the malware is saved.



Technical Details



Trojan-Dropper:W32/Blocker.A drops a program on the infected computer that "locks" the machine. Blocker.A then displays a message directing the user to send an SMS to a specified number in order to receive the password key to unlock the computer.Blocker.A may be considered Ransomware as it delivers malware that hijacks the computer system and demands the user perform a specified action in order to regain control of the machine.

Execution

Upon execution, Blocker.A drops a file, identified as Trojan:W32/Agent.AF, in the Windows temp folder. All samples analyzed so far show that the dropped file uses the filename

  • donX.tmp

where X is a number.The trojan then modifies the UserInit registry by adding a file path to the said registry so that it executes automatically.

  • HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Winlogon Userinit = %systemfolder%\userinet.exe, [Trojan:W32/Agent.AF file path]

Note: The default value of this key is Userinit = %systemfolder%\userinet.exe,

Activity

To take control of the computer, Agent.AF drop a batch file that forces the machine to reboot.During reboot, the batch file "locks" the computer screen. At the same time, it disables several Windows shortcut key combinations to prevent the user from regaining control over the machine, including CTRL+ALT+DEL, Alt+F4 and others.The trojan then displays the following message in a small window in the center:

The message directs the user to send an SMS containing a number key (generated by the malware) to the number 3649.






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