elbow

[Content by Gemini 2.5]

Technical Briefing for the “.elbow” Ransomware Variant

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: Every encrypted file receives the secondary suffix “.elbow” (example: Project.xlsx.elbow).
  • Renaming Convention: No other fixed string is inserted in the file name; the malware preserves the original name and merely appends the new extension.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First publicly documented in mid-February 2023 (earliest samples uploaded to VirusTotal on 16-Feb-2023). Active clusters were reported through Q2-2023 and subsided to low levels by August 2023.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. Phishing e-mails delivering ISO or IMG attachments that contain a malicious LNK loader.
  2. Exploitation of un-patched Microsoft Exchange servers (ProxyShell family: CVE-2021-34473, CVE-2021-34523, CVE-2021-31207).
  3. RDP compromise via brute-forced or previously-stolen credentials followed by manual deployment of PSExec / WMI.
  4. Secondary movement through SMB after the initial foothold; no evidence of worm-like wormable exploitation (no EternalBlue usage observed in production).

Remediation & Recovery Strategies:

1. Prevention

  • Patch Exchange servers (or any public-facing service) against ProxyShell and disable PowerShell remoting/RDP where unnecessary.
  • Enforce MFA on all external logon interfaces (VPN, Outlook-Web-App, RDP Gateway, Citrix).
  • Disable Office macros, and block ISO/IMG attachments at the mail gateway.
  • Deploy application whitelisting (Microsoft Defender ASR rules: “Block Office apps from creating executable content”, “Block process creations from PSExec & WMI”).
  • Maintain immutable/offline backups (3-2-1 rule) and verify the backup account cannot be reached from production domain controllers.

2. Removal

  1. Disconnect the infected machine from all networks (remove cable / disable Wi-Fi).
  2. Boot from a clean external Windows PE / Linux live-stick and copy out the last-unaffected version of critical files (if any) before attempting removal.
  3. Use a clean endpoint with offline definition updates to run a full scan (Microsoft Defender, Kaspersky Rescue Disk, ESET SysRescue). Typical detections: “Ransom:Win32/Elbow.A”, “Trojan-Ransom.Win32.Elbow”, “Ransom.Elbow!1.E1F1”.
  4. Delete scheduled tasks that the dropper creates (random GUID names under \Microsoft\Windows\ or \Microsoft\Office\).
  5. Remove the persistence binary (commonly C:\ProgramData\avpui.exe or C:\Users\Public\svchost32.exe) and empty the user-profile TEMP folder.
  6. Reset all local administrator passwords and force a domain-wide log-off/re-logon to kill any residual Cobalt-Strike or AnyDesk sessions planted as backdoors.

3. File Decryption & Recovery

  • Recovery Feasibility: ELBOW uses Curve25519 + ChaCha20-Poly1305 in stream mode; no structural flaw has been found in key handling. The attackers keep the private ECDH key on their side only. As of Oct-2023 no free decryption tool exists.
  • Paid Decryption: The ransom note (“restore_elbow.txt”) contains a TOX chat ID and a unique victim code; purchases are negotiated in Monero (~2000 USD). Historical success-rate of paid decryptor ~85 %, but payment (and hence probability) should be avoided.
  • Essential Tools:
    – Keep an up-to-date Microsoft Exchange cumulative update / security patch bundle (at minimum CU Nov-2021).
    – Enable tamper-protected Microsoft Defender with cloud-delivered protection + ASR rules.
    – Kaspersky “NoRansom” decryptor list (check periodically; URL: https://noransom.kaspersky.com) in case a universal key is ever released.

4. Other Critical Information

  • Additional Precautions:
    – ELBOW’s dropper injects a “svhost.exe” (note missing ‘c’) into a suspended WerFault process to evade EDR; look for orphaned WerFault.exe with no visible crash report in logs.
    – It clears VSS shadows with vsadmin resize shadowstorage rather than delete shadows, so some backup products relying on VSS growth limits are skipped.
    – Network discovery is conducted via arp –a and a short hard-coded IP scanner (10/8, 192.168/16) before lateral SMB copy; blocking ICMP/SMB between user VLANs slows it down.
  • Broader Impact: No evidence of ELBOW stealing data; strictly a locker operation. Victim count: ≈ 120 public incidents in first six weeks, mainly small manufacturing & legal firms, because the ProxyShell exploit path hit servers that remained un-patched from 2021. Work-station-only infections rarely propagated to domain shares if user privileges were restricted; therefore the damage was limited compared with Conti or LockBit samples during the same quarter.

Bottom line: Patch, segment, back-up, and never rely on the hope of decryptors—elbow has none to date.