Analysis by: Thea Patrice Tajonera
ALIASES:

Backdoor:MSIL/Chopper.F!dha (MICROSOFT); Backdoor.MSIL.Chopper (IKARUS)

 PLATFORM:

Windows

 OVERALL RISK RATING:
 DAMAGE POTENTIAL:
 DISTRIBUTION POTENTIAL:
 REPORTED INFECTION:
 INFORMATION EXPOSURE:

  • Threat Type: Backdoor

  • Destructiveness: No

  • Encrypted: No

  • In the wild: Yes

  OVERVIEW

Canal de infección Downloaded from the Internet, Dropped by other malware

This Backdoor arrives on a system as a file dropped by other malware or as a file downloaded unknowingly by users when visiting malicious sites. It may be hosted on a website and run when a user accesses the said website.

It executes commands from a remote malicious user, effectively compromising the affected system.

It takes advantage of certain vulnerabilities.

  TECHNICAL DETAILS

Tamaño del archivo 14,848 bytes
Tipo de archivo DLL
Residente en memoria No
Fecha de recepción de las muestras iniciales 16 Mar 2021

Arrival Details

This Backdoor arrives on a system as a file dropped by other malware or as a file downloaded unknowingly by users when visiting malicious sites.

It may be hosted on a website and run when a user accesses the said website.

Backdoor Routine

This Backdoor executes the following commands from a remote malicious user:

  • Execute arbitrary code

Other Details

This Backdoor does the following:

  • Analysis indicates this file contains log data collected from an OAB configured on a compromised Microsoft Exchange Server
  • It uses JScript to create an ASP webshell that will be executed on the target system
    • It connects to the virtual filepath ~/aspnet_client/system_web/tAbrEaGR.aspx to initialize the webshell
  • It receives code in the request body sent to the compromised web server where it is hosted

It takes advantage of the following vulnerabilities:

  • CVE-2021-26855
  • CVE-2021-26858
  • CVE-2021-27065

It requires being hosted on a web server in order to proceed with its intended routine.

  SOLUTION

Motor de exploración mínimo 9.800
Primer archivo de patrones de VSAPI 16.614.04
Primera fecha de publicación de patrones de VSAPI 24 Mar 2021
Versión de patrones OPR de VSAPI 16.615.00
Fecha de publicación de patrones OPR de VSAPI 25 Mar 2021

Step 1

Before doing any scans, Windows 7, Windows 8, Windows 8.1, and Windows 10 users must disable System Restore to allow full scanning of their computers.

Step 2

Note that not all files, folders, and registry keys and entries are installed on your computer during this malware's/spyware's/grayware's execution. This may be due to incomplete installation or other operating system conditions. If you do not find the same files/folders/registry information, please proceed to the next step.

Step 3

Scan your computer with your Trend Micro product to delete files detected as Backdoor.MSIL.CHOPPER.AA. If the detected files have already been cleaned, deleted, or quarantined by your Trend Micro product, no further step is required. You may opt to simply delete the quarantined files. Please check the following Trend Micro Support pages for more information:


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