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Added to the Dr.Web virus database: 2020-09-29

Virus description added:

  • Packer: absent


A backdoor written in C and designed to operate in the 32-bit versions of the Microsoft Windows operating systems. It allows attackers to remotely control infected computers by implementing remote shell functions—launching cmd.exe and redirecting the I/O to the attacker's C&C server. The malicious module’s original name is client_dll.dll.

Operating routine

The library has one exported function that implements the main functionality of the backdoor: ServiceMain.

At the beginning of the operation, the backdoor creates a mutex to exclude the simultaneous launch of its copy. It then decrypts the strings with an XOR operation with the byte 0x0F. List of decrypted strings:

Mozilla/4.0 (compatible; MSIE 10.0; Windows NT 6.2;+SV1;<redacted>/0xD.html
  Download OK!
  Download failed...
  Upload OK!
  Upload failed...
  Process is Killed!
  Process killed failed.
  cmd.exe /c  

The URL ns02[.]ns02[.]us/<redacted>/0xD.html is hardcoded in the body of the backdoor that locates both primary C&C servers.

After decrypting the strings, BackDoor.Remshell.24 uses the %02d%02d format to store the current minutes and seconds. These values are then used in requests to the C&C server.

Next, a separate thread is started in which, in an infinite loop, the program attempts to obtain or update the address of the second-level C&C server. When the address of the secondary C&C server is received, the backdoor starts a thread in which it sends heartbeat requests to this server.

The backdoor then periodically requests commands from the C&C server and executes them.

Obtaining the address of the secondary C&C server

To get an address, a GET request is sent to the URL specified in the configuration. In response, the server sends the string -set <arg> or -SET <arg>, where <arg> is either a number or an IP address. The resulting number is interpreted as the interval for accessing the URL specified in the configuration. If an IP address is received, the backdoor adopts it as a secondary C&C server.

It is worth noting that the thread does not stop working when it receives the valid address of the C&C server. It continues to work, which allows one to change the C&C server addresses without restarting the backdoor.

Protocol for communicating with the secondary C&C server

At the beginning of the data sent by the PUT request, the backdoor appends a header consisting of 5 bytes, which is a string formed according to the format %02d%02d. The minute and second values representing when the request was formed are substituted in this string.

With that, the request and response data are encrypted. The value of each sent byte of the request data is reduced by 0x7F, and each received byte is increased by 0x7F.

As heartbeat requests, a PUT request is sent to <cnc_addr>/webdav/0.htm with data containing the name of the infected computer and the values of the minute and second when the backdoor was launched.

To request commands from the C&C server, the backdoor sends a GET request to <cnc_addr>/webdav/O.html. It then decrypts the server’s response and parses it for commands.

Commands list

Command Description
-downloadTo download a specified file
-exitTo terminate the backdoor operation
-pskillTo terminate a specified process
-pslistTo form a list of processes
-uploadTo send a specified file to the server
othersOther commands are launched via cmd.exe /c

Responses to commands are sent by PUT requests to <cnc_addr>/webdav/<minsec>.htm, where <minsec> is the values of the minute and second when the backdoor was launched.

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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Use Dr.Web Anti-virus for macOS to run a full scan of your Mac.

After booting up, run a full scan of all disk partitions with Dr.Web Anti-virus for Linux.

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Download by serial number

  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.

Find out more about Dr.Web for Android

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