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Added to the Dr.Web virus database: 2019-05-27

Virus description added:

Packer: absent

Compilation dates:

  • 05.09.2018 05:38:37 (scansts.dll)
  • 20.12.2018 16:08:02 (CLNTCON.ocx)
  • 06.03.2019 06:58:28 (http_dll.dll)

SHA1 hashes:

  • aa0e7101b1663c23f980598ca3d821d7b6ea342d (scansts.dll)
  • b03c98a9539d4cbb17f2efc118c4b57882b96d93 (CLNTCON.ocx)
  • d52152661c836e76bebd46046ba3f877c5d381d8 (http_dll.dll)


A loader for BackDoor.PlugX.28 written in C. It is a malicious library that functions within the process of a valid executable file. This library unpacks and executes the shellcode with payload. The trojan utilizes DLL hijacking to load the malicious code into a process.

The loader’s components and the attacked applications:

Executable’s SHA-1 hash EXE DLL Shellcode
5c51a20513ba27325113d463be9b5c6ed40b5096 EMLPRO.EXE scansts.dll QuickHeal
b423bea76f996bf2f69dcc9e75097635d7b7a7aa CLNTCON.exe CLNTCON.ocx CLNTCON.ocp
5d076537f56ee7389410698d700cc4fd7d736453 EHSrv.exe http_dll.dll ESETSrv

Operating routine


Once loaded to a process, the library transfers control to the call of  scansts_2 the exported function by a hardcoded offset.

In that function the library refers to QuickHeal file, which is located at C:\Windows\System32 on the infected system. It then checks for the HKLM\Software\BINARY or HKCU\Software\BINARY registry key to determine further actions. If the registry keys are absent, the trojan initiates decryption of the QuickHeal shellcode and then calls it by passing it as an argument 0. Decryption algorithm:

s = ''
for i in range(len(d)):
    s += chr((((ord(d[i]) + 0x4f) ^ 0xf1) - 0x4f) & 0xff)


It is the improved version of scansts.dll. The main malicious code is located in the DllRegisterServer exported function. The function call decrypts the DLL’s code using an algorithm based on the XOR operation. The trojan then refers to CLNTCON.ocp and checks for the HKLM\Software\BINARY or HKCU\Software\BINARY registry keys. The shellcode is decrypted in two stages: in addition to the mentioned algorithm, the RC4 algorithm with the CLNTCON.ocp decoding key is also used.


It is equivalent to CLNCON.ocx except the following options:

  • The main trojan’s code is located in the StartHttpServer exported function;
  • ESETSrv is used as the RC4 decoding key.

QuickHeal shellcode

It is the obfuscated shellcode with an encrypted binary file and configuration. The obfuscated portion contains decryption instructions for the code that extracts the payload.


The payload is extracted by malmain function and defined by the following structure:

#pragma pack(push,1)
struct st_data
  _DWORD size;
  _BYTE data[size];
struct shellarg
  _DWORD shellcode_ep;
  _DWORD field_4;
  st_data* mod;
  _DWORD comp_size;
  st_data* cfg;
  _DWORD field_14;
  _DWORD field_18;
#pragma pack(pop)

RtlDecompressBuffer function is used for decompression. During the payload extraction process the shellcode verifies executable’s signatures. MZ and PE signatures are replaced with XV. Then DllMain is being executed. It receives the pointer to shellarg structure as a lpReserved parameter. This structure contains payload’s configuration.

Curing recommendations

  1. If the operating system (OS) can be loaded (either normally or in safe mode), download Dr.Web Security Space and run a full scan of your computer and removable media you use. More about Dr.Web Security Space.
  2. If you cannot boot the OS, change the BIOS settings to boot your system from a CD or USB drive. Download the image of the emergency system repair disk Dr.Web® LiveDisk , mount it on a USB drive or burn it to a CD/DVD. After booting up with this media, run a full scan and cure all the detected threats.
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  1. If the mobile device is operating normally, download and install Dr.Web for Android. Run a full system scan and follow recommendations to neutralize the detected threats.
  2. If the mobile device has been locked by Android.Locker ransomware (the message on the screen tells you that you have broken some law or demands a set ransom amount; or you will see some other announcement that prevents you from using the handheld normally), do the following:
    • Load your smartphone or tablet in the safe mode (depending on the operating system version and specifications of the particular mobile device involved, this procedure can be performed in various ways; seek clarification from the user guide that was shipped with the device, or contact its manufacturer);
    • Once you have activated safe mode, install the Dr.Web for Android onto the infected handheld and run a full scan of the system; follow the steps recommended for neutralizing the threats that have been detected;
    • Switch off your device and turn it on as normal.

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