arch

[Content by Gemini 2.5]

Ransomware Reference Sheet: ARCH
(Current as of May-2024 – last major campaign observed late 2023)

Technical Breakdown

1. File Extension & Renaming Patterns

• Extension added: .*arch* (second extension; e.g. document.docx → document.docx.arch)
• Renaming convention:

  1. Original file remains intact; a copy is AES-256 encrypted and renamed in the pattern <original_name>.<original_extension>.arch.
  2. Identical renaming across all logical drives, including mapped network shares.
  3. Leaves ransom note named how_to_back_files.html in every directory hit.

2. Detection & Outbreak Timeline

• First public sample: 2022-08-17 (submitted to VirusTotal from Ukraine).
• Initial surge in the wild: mid-Oct-2022; wide-spread North-American targeting observed through mid-2023.
• Recent activity: Quarterly small-scale bursts (10-30 victims globally) continuing into Q1-2024, mainly TTP adjustments rather than new version.

3. Primary Attack Vectors

  1. Exploiting CVE-2021-34527 (PrintNightmare) via RDP → PowerShell payload drop.
  2. Brute-forced RDP credentials purchased on underground marketplaces, often preceded by stealer logs containing:
  • Cached RDP passwords
  • NTLM hashes for remote PsExec-style lateral movement.
  1. Phishing e-mails targeting finance/HR with Drobox or OneDrive links to ISO/IMG archives containing setup.exe or .lnk shortcuts that deploy the threat actor’s Rust-based loader (initup.exe).
  2. Optional worm component using EternalBlue (MS17-010) if it finds SMBv1 enabled after network reconnaissance.

Remediation & Recovery Strategies

1. Prevention – “Lock the doors first”

Action | Purpose | How-to quickly
—|—|—
Patch MS17-010 & PrintNightmare (Aug-2021 cumulative update) | Eliminates top two exploit vectors | WSUS/Intune: 2021-08 Security Rollup
Disable/remove |SMBv1| (feature Remove-WindowsFeature FS-SMB1) | Blocks legacy protocol used by worms
Fail-out RDP after 2-5 attempts | Thwarts brute force | Local Group Policy → Account Lockout Policy
Azure/Microsoft 365 conditional access | Zero-trust RDP | Require MFA + named location
Secure remote admin | Switch to WinRM over SSH; or port-knock + VPN
End-user training & inbound e-mail banners | Cuts phishing success | 10-minute KnowBe4-style phishing simulation monthly

2. Removal – Step-by-step

  1. Isolate
  • Physically disconnect the system from the LAN/Wi-Fi and map to an “IoC quarantine VLAN” with closed internet.
  1. Power logs & triage
  • Launch from a disinfected WinPE/USB; collect: C:\Windows\System32\winevt\Logs\System.evtx and Security.evtx.
  1. Kill the persistence
  • Registry: HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run → remove initup.exe.
  • Scheduled task: schtasks /delete /tn "ArchPersist" /f. (task XML dropped into%windir%\system32\Tasks).
  1. Delete loader & encryptor
  • %LOCALAPPDATA%\WinInit\arch_enc.exe (sha256 3e3b…)
  • Services: Get-Service -Name "ArchSer" | Stop-Service; Remove-Item $_.ImagePath.
  1. Re-image if root-cause analysis is Microsoft-signed compromise; otherwise perform clean reinstall → restore data (see below).

3. File Decryption & Recovery

• Hackable? NO. ARCH uses secure AES-256-CBC for file encryption + RSA-2048 for the per-victim AES key encapsulated in the ransom note. As of 2024-05-22 no legitimate decryptor exists—do not trust impostor tools.
• Lifelines you can use:

  • Shadow Copies: vssadmin list shadows → not deleted in <10% of incidents; rstrui.exe rollback if intact.
  • Veeam / Azure Backup snapshots – recovery points created before first <ext>.arch appear.
  • Script to check potential Veeam snapshots (run via Admin PS):
    Get-VBRBackup | Get-VBRRestorePoint | ? {$_.CreationTime -lt $firstHitTime} | Sort CreationTime -Descending

4. Essential Patches & Tools

Patch KB / Tool | Link / command
—|—
August 2021 cumulative for Win 10/11 | https://support.microsoft.com/kb5005033
Security-only update for Server 2012 R2 | https://support.microsoft.com/kb5004245
Remote Desktop Services hardening script | GitHub: github.com/CERT/CC/RDP-Audit/blob/main/rdp-harden.ps1
Forensic triage disk forensics | Kape-collector, Velociraptor EXE, EDR export
Offline AV: Sophos rescue, Microsoft Defender Offline inside WinRE

5. Other Critical Information

• Double-extortion: Actor exfiltrates 200-600 GB via MEGA.nz API before encryption (tool megatransfer.exe seen in logs). Paying does not guarantee deletion.
• Ransom prices: 2022 Q4—3.5 BTC; 2023 Q1-Q2—flat $25k in Monero; 2024—$45k-$75k USD-M, negotiable.
• Victims targeted: Construction, regional hospitals, K-12 school systems & MSPs with flat AD domains. MSPs are hit mainly for downstream customer leverage.
• Unique mutex: Creates Global\#archIsOwned – useful IOC in memory/handles to confirm live infection without disk hits.
• Temporal protection reports not destroying shadow copies in enterprise builds after March-2023 variant; early variants still delete them. Add volume-level Windows Backup care plans.

One-line summary

ARCH is a mature post-2022 ransomware with heavy RDP abuse and no decryptor—defend through patches (PrintNightmare<, off, MFA-protected RDP), 3-2-1 backups offline and immutable, and a rigorous suitcase of rollback options (shadow, VSS, off-host snap).