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Description

CISA obtained a variant of the WHIRLPOOL backdoor. The malware was used by threat actors exploiting CVE-2023-2868, a former zero-day vulnerability affecting versions 5.1.3.001-9.2.0.006 of Barracuda Email Security Gateway (ESG).

WHIRLPOOL is a backdoor that establishes a Transport Layer Security (TLS) reverse shell to the Command-and-Control (C2) server.

For information about related malware, specifically information on the initial exploit payload and other backdoors, see CISA Alert: CISA Releases Malware Analysis Reports on Barracuda Backdoors.

Download the PDF version of this report:

For a downloadable copy of IOCs associated with this MAR in JSON format, see:

Submitted Files (1)

0af253e60456b03af49cc675f71d47b2dd9a48f50a927e43b9d8116985c06459 (ssld)

0af253e60456b03af49cc675f71d47b2dd9a48f50a927e43b9d8116985c06459

Tags

trojan

Details

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Antivirus

YARA Rules

• rule CISA_10452108_02 : WHIRLPOOL backdoor communicates_with_c2 installs_other_components
  {
     meta:
         Author = “CISA Code & Media Analysis”
         Incident = “10452108”
         Date = “2023-06-20”
         Last_Modified = “20230804_1730”
         Actor = “n/a”
         Family = “WHIRLPOOL”
         Capabilities = “communicates-with-c2 installs-other-components”
         Malware_Type = “backdoor”
         Tool_Type = “unknown”
         Description = “Detects malicious Linux WHIRLPOOL samples”
         SHA256_1 = “83ca636253fd1eb898b244855838e2281f257bbe8ead428b69528fc50b60ae9c”
         SHA256_2 = “8849a3273e0362c45b4928375d196714224ec22cb1d2df5d029bf57349860347”
     strings:
         $s0 = { 65 72 72 6f 72 20 2d 31 20 65 78 69 74 }
         $s1 = { 63 72 65 61 74 65 20 73 6f 63 6b 65 74 20 65 72 72 6f 72 3a 20 25 73 28 65 72 72 6f 72 3a 20 25 64 29 }
         $s2 = { c7 00 20 32 3e 26 66 c7 40 04 31 00 }
         $a3 = { 70 6c 61 69 6e 5f 63 6f 6e 6e 65 63 74 }
         $a4 = { 63 6f 6e 6e 65 63 74 20 65 72 72 6f 72 3a 20 25 73 28 65 72 72 6f 72 3a 20 25 64 29 }
         $a5 = { 73 73 6c 5f 63 6f 6e 6e 65 63 74 }
     condition:
         uint32(0) == 0x464c457f and 4 of them
  }

ssdeep Matches

No matches found.

Description

The file ‘ssld’ is a Linux ELF reverse shell and is a variant of WHIRLPOOL malware used on the Barracuda Email Security Gateway (ESG) device (Figure 1). The file looks for an encoded string with a ‘.io’ extension (Figure 2). The string will be decoded and the data will be passed as the C2 which will include the Internet Protocol (IP) address and port number used to establish a reverse shell.

Screenshots

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.

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.