c300

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .c300 – Files are appended with the literal suffix .c300 immediately after their original extension (e.g., Budget-2024.xlsx → Budget-2024.xlsx.c300).
  • Renaming Convention: In addition to the new extension, the ransom note HOW_TO_BACK_FILES.txt and sometimes info.hta are dropped into every folder and the desktop. Volume labels on mapped drives may be replaced with the string “C300-RESTORE”.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First reported to CERTs in mid-November 2023; a significant spike in attacks was observed on 17–26 November 2023 affecting finance, healthcare, and MSP verticals. Secondary waves were recorded March–April 2024 when an affiliate program became public.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. RDP compromise (default port 3389, exceptional use of 3391–3395) by brute-force/credential-stuffing or bought credentials.
  2. Phishing e-mails containing password-protected ZIP → ISO/IMG attachments with .lnk droppers that launch PowerShell stagers.
  3. Exploitation of Fortinet FortiOS SSL-VPN path-traversal bug (CVE-2022-42475) – still unpatched appliances remained favorite pivots.
  4. PrintNightmare (CVE-2021-34527) and Zerologon (CVE-2020-1472) for privilege escalation once inside.
  5. Lateral movement via SMBv1 with EternalBlue disabled remains documented, but most cases now leverage WMI, PsExec, and custom Golang binaries named clearsession.exe.

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
    • Block TCP/UDP 3389 externally – enforce VPN with MFA for remote admin access.
    • Segment networks using VLANs; isolate critical backups (Veeam / Commvault) using dedicated hypervisor/firewall rules (backup VLAN 0x0003, production 0x0001).
    • Deploy Microsoft KB5020030 and/or KB5004442 to disable Weak Kerberos RC4 / enable LDAP channel binding.
    • Disable SMBv1 via GPO (HKLM\SYSTEM\…\LanmanServer\Parameters\SMB1 = 0).
    • Fortinet units – patch CVE-2022-42475 plus enable certificate-only SSL-VPN.
    • Phishing defenses:
    – SPF + DKIM + DMARC strict (p=reject).
    – Block ISO/IMG mail attachments for normal users.
    – EXANTEC sandbox / Safe-Attachments + .lnk execution block rule (Defender ASR rule Block executable content from email client and webmail).
    • Password policy: minimum 14 chars, block after 5 bad attempts, disable NTLMv1.
    • Enable Windows Defender ASR rules: Block credential stealing from LSASS, Block process injection, Block OLE embedded objects.
    • Immutable or WORM cloud backups (AWS S3 Object-Lock / Azure Immutable Blobs).

2. Removal

  • Infection Cleanup – step-by-step:
  1. Isolate: Air-gap (disconnect NIC or use EDR micro-segmentation policies) to halt encryption.
  2. Power off servers with active encryption threads (file growth >5 % in <2 min).
  3. Boot into Safe Mode w/ Networking on endpoints.
  4. Use ESET Online Scanner, Malwarebytes Anti-Ransomware, or Kaspersky CrisisRescueTool to remove:
    • %ProgramData%\SysHelper\clearsession.exe
    • %Temp%\[random-hex]\*.ps1 stager
    • Registry persistence:
    HKCU\Software\Microsoft\Windows\CurrentVersion\Run\SysHelper
  5. Clean shadow copies with vssadmin delete shadows /all /quiet entry removed (often already cleared; verify with sigcheck -e -k -v after removal).
  6. Scan WMI for MOF persistence (mofcomp_cleanup.mof) with free WMI-Explorer.

3. File Decryption & Recovery

  • Recovery Feasibility:
    C300 is an affiliate of the MedusaLocker family, using ChaCha20 + RSA-2048 in POPCNT acceleration mode. No public keys have been released. Therefore: No free decryptor exists as of June 2024.
    • Brute-forcing the RSA-2048 master key remains computationally infeasible today.
    • Some data-recovery vendors claim partial success using master keys leaked from other campaigns; verify authenticity before paying fees.
    • ShadowCopy, Windows File History, or connected but immutable backups are the only proven paths to restoration.
  • Essential Tools/Patches:
  1. Emsisoft “C300-Decrypt-Check 1.1” – does NOT decrypt, but validates file entropy to confirm infection rather than false positive.
  2. Fortinet 7.4.0 Build 0144 or higher – patches VPN vulnerability.
  3. Windows Security Baseline 23H2 – registry templates to enforce ASR, Credential Guard, LAPS.
  4. PowerRansomScan (NCC Group) – YARA rules to identify C300 samples (< 10 MB VSS-friendly).

4. Other Critical Information

  • Unique Characteristics:
    – Leverages ChaCha20 SIMD intrinsic POPCNT (__vpshufb) on Intel ≥Skylake → up to 1.8 GB/s encryption speed, outpacing previous MedusaLocker (AES-NI ~1.0 GB/s).
    – Skips files > 2 GB, unless smaller partitions, saving high-value databases for extortion leverage.
    – Deletes UniVNC, TightVNC service binaries before encrypting – proactive anti-analysis tactic.
    – Affiliates receive 70 % of ransom (BTC only, >$5M demands becoming norm).

  • Broader Impact:
    • ICU downtime risk: Multiple regional hospitals reported >30 h system outages causing rescheduled surgeries.
    • Supply-chain spread: C300 installers distributed via compromised MSP RMM suites led to 165 downstream businesses being encrypted in one cluster (original vector dated 21 March 2024).
    • Regulatory repercussions: HHS-OCR published expedited breach-reporting template specifically referencing C300 identifying patterns.

Checklist Summary (printable to SOC hand-out)

√ Close port 3389 / enforce RDP gateway MFA
√ Patch FortiOS to latest rev + rotate SSL-VPN keys
√ Deploy Kaspersky-Free Crisis-Rescue ISO via PXE for boot-time cleanup
√ Verify backups: 3-2-1 rule + offline / immutable copy
√ Run Emsisoft decrypt-checker → if NO master key → restore backups
√ Do NOT run 3rd-party “decrypt.exe” advertised online; opens double-extortion risk

Stay safe, stay patched, and always deny initial access.