MAR-10459736.r1.v1 WHIRLPOOL Backdoor

August 18, 2023

MAR-10459736.r1.v1 WHIRLPOOL Backdoor
All CISA Advisories, CISA, August 18, 2023



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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 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:

AR23-230A PDF
(PDF, 357.86 KB

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

AR23-230A JSON
(JSON, 8.47 KB
Submitted Files (1)

0af253e60456b03af49cc675f71d47b2dd9a48f50a927e43b9d8116985c06459 (ssld)







Name ssld
Size 5034648 bytes
Type ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), statically linked, BuildID[sha1]=9d3200c170c74a79f66e2c885e51519866e636eb, for GNU/Linux 3.2.0, stripped
MD5 77e1e9bf69b09ed0840534adb8258540
SHA1 deadca9bd85ee5c4e086fd81eee09407b769e9b6
SHA256 0af253e60456b03af49cc675f71d47b2dd9a48f50a927e43b9d8116985c06459
SHA512 3ad6bd00c4195c9b1757a9d697196e8beffb343c331509c2eda24bbbd009cc1af552a1900ab04d169a22d273e6359cb2ff149050a7f792b9630108a4af226e2d
ssdeep 98304:1z2EGoxipg0NPbuqbVxbNgqE+Q+F4YGZLx4BAFm/CyU:LLXYGNFLj
Entropy 6.385269
Malware Result unknown
ESET a variant of Linux/WhirlPool.A trojan
YARA Rules
  • rule CISA_10452108_02 : WHIRLPOOL backdoor communicates_with_c2 installs_other_components
           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”
           $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 }
           uint32(0) == 0x464c457f and 4 of them
ssdeep Matches

No matches found.


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.













Figure 1. – The reverse shell component of ‘ssld’.










Figure 2. – The file ‘ssld’ looking for a string with a ‘.io’ extension.


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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