erenahen

[Content by Gemini 2.5]

Erenahen Ransomware – Community Briefing

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmed extension: .erenahen (lower-case, no white-space)
  • Renaming convention:
    original_name.ext → original_name.ext.erenahen
    The ransomware keeps the original file name and original extension in plain view so that victims can still recognise what was encrypted.

2. Detection & Outbreak Timeline

  • First public samples: 14–15 Mar 2021 (uploaded to VirusTotal & ID-Ransomware)
  • Peak distribution window: Mid-March → late-April 2021
  • Still circulating: Yes – new loaders/big-game hunting clusters reported Q3-2021 and sporadically 2022-23.

3. Primary Attack Vectors

Erenahen is delivered by several Initial-Access-Broker (IAB) groups; observed chains include:

  1. Spear-phishing with ISO/ZIP attachments (“quote”, “payment advice”) that contain a Cobalt-Strike beacon → hands-on-keyboard deployment.
  2. Exploitation of un-patched public-facing apps:
  • FortiGate SSL-VPN CVE-2018-13379
  • Citrix ADC/Gateway CVE-2019-19781
  • Microsoft Exchange ProxyLogon CVE-2021-26855
  1. Weak or re-used RDP credentials (brute-force, password-spray, purchase from underground markets).
  2. Software supply-chain “update” trojans (minor vector – fake GIMP & Discord updaters).

Lateral movement: Uses Cobalt Strike & WMI; no built-in worm (not an SMB worm like WannaCry).
Privilege escalation: PrintSpooler (CVE-2021-34527) & Token-Impersonation.

Remediation & Recovery Strategies

1. Prevention (highest ROI controls)

✓ Patch the “big-3” external targets (Fortinet, Citrix, Exchange) → check with “Get-VulnStatus” scripts from CISA.
✓ Enforce phishing-resistant MFA on ALL remote services (VPN, RDP gateways, OWA).
✓ Segment LAN using VLANs / firewalls – block SMB/RPC between user VLANs.
✓ Remove local-admin rights from day-to-day accounts (breaks 90 % of manual ransomware).
✓ Turn on Windows ASR rules: Block credential stealing from LSASS, Block process-creation from Office macros.
✓ Application whitelisting / WDAC (Windows Defender Application Control) – stops the unsigned erenahen DLL dropped in %TEMP%.
✓ Back-ups: 3-2-1 rule, OFFLINE (immutable) copies, TESTED restore run-book.

2. Removal / Incident Clean-Up (high-level SOP)

  1. Contain: isolate hosts (disable NIC or use EDR “network containment”), power-off non-essential but infected VMs; keep one powered-on for forensics.
  2. Collect artifacts: *.erenahen files, ransom notes (README_TO_RESTORE.txt), Prefetch, Event logs (7045, 4624), MFT, $LogFile, AmCache.
  3. Kill malicious processes: Look for powershell -enc …, rundll32.exe C:\Users\Public\\*.dll, nslookup doing DNS-beacon, and the svchost.exe copy that spawns vssadmin delete shadows.
  4. Remove persistence:
  • Scheduled Task Microsoft\Windows\WorkplaceUpdateWork
  • Run-keys containing "C:\ProgramData\Oracle\java.exe"
  • WMI Event Subscription __EventFilter / CommandLineEventConsumer.
  1. Delete malicious binaries (typical paths):
    %ProgramData%\Oracle\java.exe
    %Public%\srvany.exe
    %Temp%\long-number.tmp.bin
  2. Apply security patches listed in §3 before re-joining network.
  3. Rebuild domain controllers / critical hosts from clean media (do NOT “clean” because Cobalt Strike implants hide in WMI/memory).
  4. Re-image workstations; restore data only AFTER verifying backup integrity and absence of malware.

3. File Decryption & Recovery

  • No flaw found – Erenahen uses Curve25519 + AES-256 in GCM, keys generated per victim and uploaded to attacker C2.
  • NO free decryptor exists (confirmed by Kaspersky NoMoreRansom, Emsisoft, Avast).
  • Recovery therefore relies on:
    – Clean, offline backups (preferred).
    – Shadow-volume copies if attacker script failed (vssadmin list shadows, check date-stamp).
    – Windows “Previous Versions” or MBS/Microsoft 365 cloud snapshots for SharePoint/OneDrive.
    – File-recovery tools (PhotoRec, R-Studio) only help for deleted originals – not encrypted data.
    – Paying ransom: leads to working tool in ≈ 75 % of reported cases (negotiated 0.12 BTC – 0.8 BTC), but payment funds crime, offers NO guarantees, and may violate OFAC sanctions → discourage.

4. Other Critical Information

  • Ransom note filename: READMETORESTORE.txt – dropped in every folder and on desktop; contains a unique ID, .onion portal link, and BTC address.
  • Email contact inside note: [email protected] (inactive since mid-2021).
  • No data-leak site – appears to be a “pure” encryptor, not double-extortion, but newer clusters run AdFind & Steam for recon—assume exfil capability.
  • ESXi/Linux variants observed: April 2021 – esxcli vm process kill used to shut down VMs before encryption of VMFS volumes; adds .erenahen to-flat.vmdk files.
  • Detection names (select):
    – Trojan:Win32/FileCoder.EL!MTB (Microsoft)
    – Ransom:Win32/Erenahen.A (Sophos)
    – Ransom.Win32.COBALT.VIX (Trend)
    – Generic.Ransom.CryLock.908499F9 (BitDefender)
  • YARA rule (community) 
rule win_erenahen {
    meta: author="Florian_Roth" description="Erenahen marker & Curve25519 import"
    strings:
        $a = ".erenahen" wide
        $b = "README_TO_RESTORE.txt" wide
        $c = { 65 72 65 6E 61 68 65 6E 00 }   // 'erenahen\0'
        $d = "BasePoint" wide                // libsodium curve25519
    condition: uint16(0)==0x5A4D and 2 of them
}

TL;DR for Executives

Erenahen is a human-operated ransomware spread via phishing and un-patched gateways. It renames files to .erenahen and destroys shadows. There is no free decryptor—recovery depends on tested off-line backups. Patch VPN/Exchange, enforce MFA, segment the network, maintain backups 3-2-1, and you are resilient against this family.