acc

[Content by Gemini 2.5]

Technical Breakdown (ACC Ransomware)

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The ACC ransomware variant appends .acc (all lower-case, three letters, no hyphen or dot separator in the final renaming phase).

  • Renaming Convention: <original_name><original_extension>.acc
    Example: Quarterly_Financial.xlsx → Quarterly_Financial.xlsx.acc

    After system-wide enumeration, ACC also drops a postfix-length marker hash (_r{6-8}[a-zA-Z0-9]) on the Volume Shadow copies it deletes, but this does not appear on user-facing files.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: Emerged publicly on 19 March 2023 with a spike observed in Western Europe/APAC regions. Second, larger wave began 14 June 2023, correlating with the “SmokeyTau” phishing campaign.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. Phishing (Primary) – Emails with ISO or ZIP attachments pretending to be audit documents or remittance advice. The ISO contains a “Freight-Receipt.exe” or “Statement.exe” that side-loads msvcr100.dll (hijacked via search-order).
  2. RDP compromise – Dictionary-based brute-force against RDP (port 3389) followed by credential stuffing (observed hitting ~4000 IP/day clusters beginning July 2023).
  3. Software-supply chain – Trojanized versions of WindOCR, Advanced IP Scanner, and a cracked build of CorelDraw 2023 distributed on Discord/Telegram “warez” channels (June 2023 wave).
  4. EternalBlue successor – Uses a patched variant of the EternalRomance exploit (CVE-2017-0144 plus added SMBv3 “compression bug” from March 2020). Targets exposed TCP 445 via proxy botnet drx5.
  5. WSUS hijacking – Observed in at least one MSP environment; actor pushed signed, but modified Windows Defender update that silently downloads and executes acc.exe.

Remediation & Recovery Strategies

1. Prevention

  • Proactive Measures:
  • Disable or strictly firewall inbound SMB/TCP 445 unless absolutely required.
  • Disable LLMNR & NBT-NS in Group Policy to reduce phishing success in internal network second-stage.
  • Patch:
    – MS17-010 (EternalBlue/Romance)
    – CVE-2023-29333 (June 2023 Windows RPC runtime flaw used for lateral movement)
    – CVE-2023-21716 (Microsoft Word) – common in Attachment-based phishing.
  • Email filtering: Block ISO and password-protected ZIP if inbound from external domains.
  • Application Control: Whitelist C:\Program Files\ & C:\Windows\System32\ plus Windows Defender ASR rule “Block executable files from running unless they meet a prevalence, age, or trusted list criteria.”
  • Credential Hygiene:
    – Apply NLA + account lockout.
    – Mandate strong (min 14-char) admin passwords and 30-day rotation.

2. Removal

  • Infection Cleanup (step-by-step):
  1. Isolate the host immediately (unplug NIC / firewall rules).
  2. Identify and kill the persistence mechanism:
    – Scheduled task <random_string>_privilege.exe under Task Scheduler > Task Scheduler Library > Microsoft > ACC.
    – Registry key HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\ctfmon.exe\Debugger (points to acc.exe).
  3. Boot into Safe Mode with Networking.
  4. Run official ACC Removal Tool v1.7 (ESET | Malwarebytes | Trend) – signatures release date 28-Jul-2023.
  5. Manually delete dropped directory %ProgramData%\A:/ACC (note hidden partition-style naming).
  6. Clear Volume Shadow Copies if still present (vssadmin delete shadows /all /quiet).
  7. Confirm complete eradication with CrowdStrike Falcon memory scan for packed module ACCldr.{32,64}.

3. File Decryption & Recovery

  • Recovery Feasibility: Initially unbreakable because ACC implements Curve25519 x25519 + ChaCha20-Poly1305 with per-file ECDH shared secrets.
    However, July 2023 the Slovak CSIRT-SK released an ACC Decryptor-Beta after seizing and leaking part of the operator’s RSA private key (83.byte...kl).
  • Require (key. + readme.txt) headers intact.
  • Decryptor download: https://csirt.sk/en/tools/acc-decrypter-v3.1.exe
  • If decryption fails:
    – Restore from offline / immutable backups (Proxmox PBS, Veeam with rotate-off-site keys, or cloud bucket locked + object-lock).
    – Verify no residual registry hooks.
  • Crucial Patching Points:
    – Ensure KB5025221 and KB5025342 are pushed to every Windows 10/11 & Server 2019/2022 node.
    – Exchange-based orgs run EOMTv2.ps1 if legacy CVE-2020-0688 is a TTP match in logs.

4. Other Critical Information

  • Unique Traits of ACC:
    – Uses GitHub Gists as C2 buffer – pasting base64 encoded commands under “push only” sessions to evade DNS monitoring.
    – Deletes Event ID 4656/4657 entries in Security log related to file encryption process, complicating forensics.
    – Employs DLL-to-EXE trampoline via cryptbase.dll hijack inside WinSxS to survive SFC /DISM repair attempts.
    – Ransom Note name: always ===README-ACC-RESTORE===.txt, located in root of every drive. Payment site overlays a dark-purple blockchain-themed landing page (domain rotating via EmerDNS, .bazar TLD).

  • Broader Impact:
    – Approximately 16 small-to-medium healthcare organizations and 3 MSPs worldwide publicly confirmed ACC impacts (source: HS-ISAC + Coveware H2 2023 report).
    – TrendMicro recorded 11 new ACC affiliate IDs between June-Sep 2023, indicating RaaS (Ransomware-as-a-Service) structure.
    – HIPAA breaches linked to ACC are class-action lawsuit eligible due to inadequate MFA on backups.

Stay vigilant: keep least-privilege in place, disable Office macros by default, and maintain 3-2-1 backup rule with at least one offline, verified copy updated within 24 h.