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Backdoor.ASP.WEBSHELL.SMB

May 29, 2021
 Analysis by: Maria Emreen Viray
 ALIASES:

Exploit:ASP/CVE-2021-27065.B!dha (MICROSOFT)

 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

Infection Channel:

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.

  TECHNICAL DETAILS

File Size:

2,393 bytes

File Type:

ASP

Memory Resident:

No

Initial Samples Received Date:

29 May 2021

Payload:

Others

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:

  • saves a file

Other Details

This Backdoor does the following:

  • It is a modified Microsoft Exchange Offline Address Book (OAB) configuration file
    • ExternalUrl parameter was maliciously altered to directly check for a file uploaded and saves it locally as the following ASPX file:
      • error.aspx

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

It takes advantage of the following exploit(s):

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

  SOLUTION

Minimum Scan Engine:

9.800

FIRST VSAPI PATTERN FILE:

16.602.05

FIRST VSAPI PATTERN DATE:

18 Mar 2021

VSAPI OPR PATTERN File:

16.603.00

VSAPI OPR PATTERN Date:

19 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.ASP.WEBSHELL.SMB. 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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