Threat Descriptons



Category :


Type :


Aliases :

Trojan-Dropper:W32/Blocker.A, Trojan.Dropper.Blocker.A


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


Based on the settings of your F-Secure security product, it will either move the file to the quarantine where it cannot spread or cause harm, or remove it.

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 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.

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.


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,


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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