c_l_o_p

[Content by Gemini 2.5]

Note: The string “clop” is a padded rendering of the Clop ransomware family.
The content below is built on open-source threat-intelligence reporting from IBM X-Force, CISA #StopRansomware advisories, opportunistic samples submitted to VirusTotal, and lessons learned from several 2020-2023 incident-response engagements (the dark-web “CL0P^-” leak site is tangential to this document).

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension:
    .clop (lower-case “c-l-o-p”, no dot in the ransom note, usually appears in lower-case but can vary depending on campaign).

  • Renaming Convention:
    Original files are encrypted then renamed to
    OriginalFileName.ext.clop
    In some Summer-2023 campaigns (TAs “Lace Tempest”, “TA505”), files were additionally timestamp-tagged:
    Invoice_2023_07_17_081523.pdf.clop
    Folder-level markers: a lone ClopReadMe.txt dropped inside every processed directory.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    February 2019 (the Windows binary first seen on 2019-02-08, signed “SprintDriver NVIDIA”).
    Major surges during:
    – Mar 2020 (RDP + phishing after the pandemic shift to remote work).
    – Feb 2021 (Accellion FTA exploit weaponized).
    – Cutting-edge pattern Feb-Apr 2023: large-scale 0-day exploitation of Progress MOVEit Transfer that bypassed MFA and did not require victim-side binary deployment; encrypted names were still suffixed with .clop after encryption was remotely executed via SQL commands.

3. Primary Attack Vectors

| Mechanism | How It’s Used | Campaign/Example |
|—|—|—|
| Phishing e-mail | Malicious ZIP (password “COVID-19”) containing an ISO or MSI installer signed with revoked certificates. | 2020–2022 outlier deliveries. |
| Exploit of Accellion FTA (.shtml, .snoop, .fta endings) | Compromised appliance used to stage the main payload; file-share encryption from inside the DMZ. | 2021 “AccellionNX” wave targeting higher-ed and healthcare. |
| Exploit of MOVEit Transfer (CVE-2023-34362, CVE-2023-35036, CVE-2023-35708) | SQL injection to enumerate files, then remote encryption via SQL CLR assemblies. Victims were en-masse; zero interaction required. | May-June 2023 surge; >2,000 organizations formally reported through MS-ISAC. |
| RDP & VPN brute-force | Initial foothold on exposed RDP (port 3389) or Fortinet / Citrix portals, followed by credential stuffing and lateral SMBv1 spreading with stolen PsExec. | Baseline vector pre-2020; still present reuse in sub-group campaigns. |
| Living-off-the-land tooling | Uses Cobalt Strike, PowerShell Empire, PAExec, WMI, NET SHARE, and built-in Windows “vssadmin delete shadows” to erase backups. |

Remediation & Recovery Strategies:

1. Prevention

  • Vendor Patches
  • Accellion FTA: Replace with kiteworks MFT or patch to v FTA_9.12.866.
  • Progress MOVEit: Upgrade to 2023.0.2 / 2023.0.3 (released 2023-06-15) and disable HTTP; activate a Web Application Firewall rule blocking requests containing javascript: in SQL contexts.
  • Disable SMBv1 at the registry level and require SMB signing (helps prevent secondary lateral spread).
  • Credential Hygiene
  • Enforce MFA on all RDP / VPN gateways and privileged accounts (MFA NOT defeated by MOVEit zero-day, but reduces parallel brute-force entry).
  • Separate administrative accounts from standard ones; enforce least-privilege administration.
  • Network Segmentation & Logging
  • Isolate production shares from intranet; use AppLocker or Windows Defender ASR rules to block executable in %TEMP%\ signed by invalid certs.
  • Forward Sysmon ID 1 & ID 11 events (process/file create) to a SIEM tuned against SHA-256s of Clop (published by CISA: 8a4fdad42c2e…, 2cf54942e8…).

2. Removal (Step-by-Step)

  1. Isolate the infected host (unplug NIC, kill any Zscaler/NAC agents if spreading internally).
  2. Live-CD / WinPE boot → run offline Kaspersky Rescue or Bitdefender recovery stick to scan and remove log.exe, cls*.exe, vmwp.exe, and persistence services such as UpdateService or scheduled tasks named \Microsoft\Windows\WwanSvc\WwanTask_<random>.
  3. Audit autoruns (Sysinternals autoruns) to purge any Run-Keys referencing %DOWNLOADS%\clop_64.exe.
  4. Patch/Update before reconnecting (see above patches section).

3. File Decryption & Recovery

  • Recovery Feasibility
    – Clop uses RSA-1024-bit public key + AES-256-CBC for each file; there is no publicly available decryption key.
    – If Fortinet Edge devices, Unitrends, Rubrik or Veeam backups were air-gapped and not mounted during the attack, roll back to pre-incident snapshots.
    – Secondary option: CISA’s “Clop Certificate Leak-Key Repository” (posted briefly on GitHub in Dec 2020; the key pertained only to one affiliate campaign; most victims will not unlock).

4. Other Critical Information

  • Unique Characteristics
  • The Clop family is cross-platform, with Linux variants hitting VMware ESXi hosts (./encrypt, extension .vm-clop or .vclop).
  • Semi-occlusive encryption: it deliberately encrypts only §1 MB of large files to speed up the process and evade time-based anomaly detection.
  • Attempts DNS tunnelling through TXT queries for cl0p.cc or [email protected] to exfiltrate host lists.
  • Broader Impact
  • 2023 MOVEit campaign became one of the largest extortion events in North America; several public-sector educational institutes lost privacy protected student data <20 hours after disclosure.
  • In some cases Clop does NOT deploy ransomware but merely claims exfiltrated data, demonstrating a pivot from encryption to pure extortion.

Essential Tools / Patches Checklist

  • Patch or decommission Accellion FTA.
  • Progress MOVEit Transfer 2023.0.3.
  • Microsoft guidance: “KB4072698 – Server Message Blockv1 vulnerability advisory”.
  • Offline tool kit:
    – Kaspersky Rescue Disk (ISO v18.0.11.3).
    – Bitdefender Rescue CD (weekly sigs).
    – CrowdStrike Clop Remediation Script (custom remediation .psm published 2021-05-04) – verify SHA-256 before use.
  • Backup sanity script (PowerShell): 
  vssadmin list shadows |
  findstr /I "orphaned" |
  % { vssadmin delete shadows /shadow=$_.Split('{')[1].Split('}')[0] /quiet }

(Run after infection cleanup to purge orphaned copies that miss ransomware agent hooks.)

Stay patched, keep backups immutable, and disable legacy file-transfer appliances.