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Summary

Description

CISA received three files for analysis. The files included three webshells written in PHP: Hypertext Preprocessor (PHP), Active Server Pages Extended (ASPX), and .NET Dynamic-Link Library (DLL). The sample “sd.php” is highly obfuscated and uses rot13 algorithm, zlib for compression and base64 encoding for obfuscation. The “osker.aspx” webshell code was padded with junk code. The .NET DLL webshell is a .NET compiled version of osker.aspx. The samples are interactive webshells and have the ability to upload and manage files, create directories and files, and execute commands on the target machine.

For more information about this compromise, see Joint Cybersecurity Advisory Threat Actors Exploit Progress Telerik Vulnerabilities in Multiple U.S. Government IIS Servers

Download the PDF version of this report:

For a downloadable copy of IOCs, see below or the JSON file.

Submitted Files (3)

6ce087b904af8a01aae73ac77d81822ad41799f89a5d301dce45191c897012aa (osker.aspx)

b63c95300c8e36b5e6d3393da12931683796f88fd4601ba8364658b4d12ac05b (App_Web_jl37rjxu.dll)

ea98368f6ecb5281654a6a9e4c649ef9b53860f1ee32340145b61e0e42e1072a (sd.php)

Findings

ea98368f6ecb5281654a6a9e4c649ef9b53860f1ee32340145b61e0e42e1072a

Tags

obfuscatedtrojanuploaderwebshell

Details

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Antivirus

YARA Rules

• rule CISA_10443863_01 : backdoor remote_access_trojan webshell exploitation information_gathering remote_access accesses_remote_machines anti_debugging captures_system_state_data controls_local_machine compromises_data_availability compromises_data_integrity fingerprints_host installs_other_components
  {
     meta:
         Author = “CISA Code & Media Analysis”
         Incident = “10443863”
         Date = “2023-05-11”
         Last_Modified = “20230522_1200”
         Actor = “n/a”
         Family = “n/a”
         Capabilities = “accesses-remote-machines anti-debugging captures-system-state-data controls-local-machine compromises-data-availability compromises-data-integrity fingerprints-host installs-other-components”
         Malware_Type = “backdoor remote-access-trojan webshell”
         Tool_Type = “exploitation information-gathering remote-access”
         Description = “Detects obfuscated and deobfuscated interactive PHP webshell samples”
         SHA256 = “ea98368f6ecb5281654a6a9e4c649ef9b53860f1ee32340145b61e0e42e1072a”
     strings:
         $e0 = { 65 76 61 6c }
         $e1 = { 72 6f 74 31 33 }
         $e2 = { 62 61 73 65 36 34 }
         $e3 = { 67 7a 69 6e 66 6c 61 74 65 }
         $e4 = { 73 68 65 6c 6c }
         $e5 = { 78 61 69 73 79 6e 64 69 63 61 74 65 }
         $e6 = { 54 75 62 61 67 75 73 4e 4d }
         $s0 = { 58 30 4d 42 31 33 }
         $s1 = { 74 75 6e 61 66 65 65 73 68 }
         $s2 = { 70 61 73 73 77 6f 72 64 }
         $s3 = { 6f 6e ( 63 | 43 ) 6c 69 63 6b 3d }
         $s4 = { 6a 61 76 61 73 63 72 69 70 74 3a 78 79 6e }
     condition:
         (6 of ($e*)) or (3 of ($s*))
  }

ssdeep Matches

No matches found.

Description

This sample is an obfuscated PHP interactive webshell. This webshell is encoded and obfuscated using rot13, gzinflate and base64 as seen in the following code: “eval(str_rot13(gzinflate(str_rot13(base64_decode(($sym))))));” The obfuscated code is a string and is stored in the $sym variable from where it is read and decoded upon execution (Figure 1). The webshell requires the password “pass” for authentication and uses the string “$xyn=’tunafeesh’;” as a cookie to authenticate.

This webshell enumerates the local system it infects including the operating system, current user, directories, files and permissions. The webshell has the ability to create, rename, and delete files and directories. Furthermore, it has the ability to upload additional files to the affected webserver, run in Safe Mode and execute commands via cmd.exe (Figure 2). The webshell provides a Graphical User Interface (GUI) to the operator to perform these operations on the infected machine.

—Notable Strings Begin—
eval(str_rot13(gzinflate(str_rot13(base64_decode(($sym))))));
tunafeesh
pass
TubagusNM
xaisyndicate
garuda tersakti
con7ext_shell
b374k shell
X0MB13
X0MB13@REBORN.COM
hxxp[:]//www[.]twitter[.]com/X0MB13_
hxxp[:]//www[.]fb[.]com/xombie.xombie.7
onClick=”xyn
—Notable Strings End—

Screenshots

Figure 1. – $sym variable with obfuscated code.

Figure 2. – sd.php webshell interface. Threat Actor (TA) would have access to this interface remotely to conduct various actions like upload additional files, create directories and files, run commands and more.

6ce087b904af8a01aae73ac77d81822ad41799f89a5d301dce45191c897012aa

Tags

backdoor trojan webshell

Details

–>

Antivirus

YARA Rules

• rule CISA_10443863_02 : backdoor remote_access_trojan webshell exploitation information_gathering remote_access accesses_remote_machines anti_debugging captures_system_state_data controls_local_machine compromises_data_availability compromises_data_integrity fingerprints_host installs_other_components
  {
     meta:
         Author = “CISA Code & Media Analysis”
         Incident = “10443863”
         Date = “2023-05-11”
         Last_Modified = “20230522_1200”
         Actor = “n/a”
         Family = “n/a”
         Capabilities = “accesses-remote-machines anti-debugging captures-system-state-data controls-local-machine compromises-data-availability compromises-data-integrity fingerprints-host installs-other-components”
         Malware_Type = “backdoor remote-access-trojan webshell”
         Tool_Type = “exploitation information-gathering remote-access”
         Description = “Detects interactive ASP NET webshell samples”
         SHA256 = “ea98368f6ecb5281654a6a9e4c649ef9b53860f1ee32340145b61e0e42e1072a”
     strings:
         $s0 = { 3c 25 40 20 50 61 67 65 20 4c 61 6e 67 75 61 67 65 3d 22 43 23 22 }
         $s1 = { 62 61 73 65 36 34 ( 44 | 64 ) 65 63 6f 64 65 }
         $s2 = { 53 65 6c 65 63 74 20 2a 20 66 72 6f 6d 20 57 69 6e 33 32 5f 50 72 6f 63 65 73 73 }
         $s3 = { 53 45 4c 45 43 54 20 2a 20 46 52 4f 4d }
         $s4 = { 73 71 6c 63 6d 64 2e 65 78 65 }
         $s5 = { 63 6d 64 2e 65 78 65 }
         $s6 = { 49 49 53 20 56 65 72 73 69 6f 6e }
         $s7 = { 43 72 65 61 74 65 4e 6f 57 69 6e 64 6f 77 }
     condition:
         all of them
  }

ssdeep Matches

No matches found.

Relationships

Description

This sample is an ASP .NET webshell. The webshell code was padded with junk code for detection evasion. The beginning of the webshell code can be seen in Figure 3. It is possible to access the webshell interactively via browser to view the GUI as seen in Figure 4.

This webshell has the ability to enumerate drive name and type, software, operating system versions, processes, and users, and has ability to copy, create and delete files, directories and databases. Furthermore, this webshell is able to upload, download, run and execute commands using cmd.exe and sqlcmd.exe. This webshell has the ability to interact with and manipulate SQL databases. Furthermore, this webshell uses Windows Management Instrumentation (WMI) Management Objects to query processes, users and network domains. It is also able to encode and decode data using base64.

—Notable Strings Begin—
osker
321
base64Decode
Select * from Win32_Process
Select * from Win32_Process Where ProcessID
Add_Table_Row(tbl, “Server IP”, Request.ServerVariables[“LOCAL_ADDR”]);
Add_Table_Row(tbl, “Host Name”, Dns.GetHostName() );//Environment.MachineName);
Add_Table_Row(tbl, “IIS Version”, Request.ServerVariables[“SERVER_SOFTWARE”]);
Add_Table_Row(tbl, “IIS APPPOOL Identity”, Environment.UserName);
Add_Table_Row(tbl, “OS Version”, Environment.OSVersion.ToString());
myconn = new SqlConnection(connections.Text);
myconn.Open();
string command = query;
mycomm = new SqlCommand(command, myconn);
SqlDataReader dr = mycomm.ExecuteReader();
string query = “Select * from Win32_Process Where ProcessID = “” + processName + “””;
ManagementObjectSearcher searcher = new ManagementObjectSearcher(query);
ManagementObjectCollection processList = searcher.Get();
ManagementObjectSearcher QS=new ManagementObjectSearcher(new SelectQuery(query));
—Notable Strings End—

Screenshots

Figure 3. – Beginning of osker.aspx webshell code.

Figure 4. – Web interface for osker.aspx webshell. The webshell interface password is “321”.

b63c95300c8e36b5e6d3393da12931683796f88fd4601ba8364658b4d12ac05b

Tags

backdoortrojanwebshell

Details

–>

Antivirus

YARA Rules

• rule CISA_10443863_03 : backdoor remote_access_trojan webshell exploitation information_gathering remote_access accesses_remote_machines anti_debugging captures_system_state_data controls_local_machine compromises_data_availability compromises_data_integrity fingerprints_host installs_other_components
  {
     meta:
         Author = “CISA Code & Media Analysis”
         Incident = “10443863”
         Date = “2023-05-16”
         Last_Modified = “20230605_1500”
         Actor = “n/a”
         Family = “n/a”
         Capabilities = “accesses-remote-machines anti-debugging captures-system-state-data controls-local-machine compromises-data-availability compromises-data-integrity fingerprints-host installs-other-components”
         Malware_Type = “backdoor remote-access-trojan webshell”
         Tool_Type = “exploitation information-gathering remote-access”
         Description = “Detects .NET DLL webshell samples”
         SHA256 = “b63c95300c8e36b5e6d3393da12931683796f88fd4601ba8364658b4d12ac05b”
     strings:
         $s0 = { 53 00 65 00 6c 00 65 00 63 00 74 00 20 00 2a 00 20 00 66 00 72 00 6f 00 6d 00 20 00 57 00 69 00 6e 00 33 00 32 00 5f 00 50 00 72 00 6f 00 63 00 65 00 73 00 73 }
         $s1 = { 62 61 73 65 36 34 ( 44 | 64 ) 65 63 6f 64 65 }
         $s2 = { 53 00 45 00 4c 00 45 00 43 00 54 00 20 00 2a 00 20 00 46 00 52 00 4f 00 4d }
         $s3 = { 49 00 49 00 53 00 20 00 41 00 50 00 50 00 50 00 4f 00 4f 00 4c }
         $s4 = { 4d 61 6e 61 67 65 6d 65 6e 74 4f 62 6a 65 63 74 }
         $s5 = { 43 72 65 61 74 65 4e 6f 57 69 6e 64 6f 77 }
         $s6 = { 73 71 6c 71 75 65 72 79 }
     condition:
         all of them
  }

ssdeep Matches

No matches found.

Relationships

Description

This is a 32-bit .NET Dynamic-Link Library (DLL) file. This sample is a ASP .NET webshell and is related to the osker.aspx file. These webshells may affect Microsoft Exchange Servers and IIS services exploited by the ProxyLogon vulnerability. This sample is a .NET DLL file that is created by the ASP.NET Runtime when ASPX script is seen for the first time on the system. The capabilities and functions are identical to the osker.aspx file.

Relationship Summary

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

• 1-888-282-0870
• CISA Service Desk (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/

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:

• 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.