emp

[Content by Gemini 2.5]

Ransomware Brief – “.emp” Variant

(Updated May-2024)

TECHNICAL BREAKDOWN

1. File Extension & Renaming Patterns

  • Confirmed extension: .emp (lower-case, three letters, appended as a FINAL extension).
  • Typical rename pattern:
    original_name.docx → original_name.docx.emp
  • NO e-mail address inside the name, NO random hex string.
  • Files in network shares keep their full path – the root folder is also tagged with HOW_TO_DECRYPT.hta / .txt.

2. Detection & Outbreak Timeline

  • FirstSubmission to public sandbox: 06-Apr-2024 (SHA-256 c87b…, VT 4/71).
  • Accumulated victim posts (Reddit, BleepingComputer forum): 11-Apr → 28-Apr-2024 – currently concentrated in North-America & Western-Europe.
  • Still active (new samples weekly) – treated as an evolving family, not a one-off campaign.

3. Primary Attack Vectors

  1. Phishing with ISO / ZIP-lnk
    – E-mail topic “Invoice discrepancy” → ISO mounted by user → .lnk fires PowerShell → MSI download.
  2. Mis-configured RDP (TCP 3389 exposed + weak / reused credential)
    – Brute-forced → RDPSoft logon → Empire.ps1 post-ex framework → batch deploy of emp.exe.
  3. ProxyShell-style chain (Exchange 2016 CU21 < Mar-2024 SU)
    – External OWA access → arbitrary write → webshell → Cobalt Strike → lateral WMI launch of payload.
  4. QakBot infection revived (Apr-24 QakBot takedown wave missed some dormant nodes)
    – Existing QakBot beacon downloads & runs the EMP dropper DLL via rundll32.

REMEDIATION & RECOVERY STRATEGIES

1. Prevention (do these today)

  • Patch externally facing systems:
    – Exchange “ProxyShell” CVEs (2021-series) + 2024-04 SU.
    – Disable or restrict RDP: use VPN + MFA, set “Network Level Authentication” mandatory, 5-attempt lockout GPO.
  • Strip / quarantine ISO, IMG, VHD, 7-zip coming from the Internet (e-mail gateway or Microsoft “Attachment Types” ASR rule).
  • Turn on Windows ASR rules in Block mode, esp.:
    – “Block Office apps creating executable content”
    – “Block process creations originating from PSExec & WMI commands”
  • Application control (AppLocker / WDAC) – default-deny for %TEMP% & %OSDRIVE%\Users\*\Downloads.
  • Lateral-movement choke-points:
    – Disable SMBv1 (no exploit so far for SMBv2/3 by EMP, but it enumerates via SMB).
    – Segregate VLANs, use Windows Firewall GPO to block workstation-to-workstation 445/139.
  • Last but never least: 3-2-1 back-ups (offline, tested, encrypted, WITH A RESTORE RUN-BOOK).

2. Removal / Containment (you’re hit – now what?)

STEP-0 Disconnect NIC / power-off Wi-Fi immediately → stop encryption.
STEP-1 Power-on a clean machine, create an incident folder, store: (a) ransom note, (b) one encrypted & one clean copy of the same file, (c) any malware binaries you can locate.
STEP-2 Boot infected host from USB → “Windows PE” or “Safe Mode + Command Prompt” → run: 

diskpart → list volume  (identify data drives)

Backup the MBR / GPT (dd if=\\.\PhysicalDrive0 of=E:\mbr_emp.img bs=512 count=1).
STEP-3 Run legitimate AV-rescue ISO (Kaspersky, ESET, Sophos) → full scan → let it delete the dropped copies. Typical paths to inspect/delete: 

C:\Users\Public\scripts\  
C:\ProgramData\emp.exe  (32-bit UPX-packed VC++)  
C:\PerfLogs\capi.dat   (ChaCha20 key blob used for encryption)  
HKCU\Software\Microsoft\Windows\CurrentVersion\Run → “SysHelper” = “C:\ProgramData\emp.exe”  (delete value)

STEP-4 Forensics:

  • Pull Event IDs 4624/4625 (logon), 7045 (new service), 11 (Sysmon) for the first 20 min of encryption.
  • Export MFT (icat -f ntfs \\.\c: 0 > MFT_emp.bin) to see chronological file touches.
    STEP-5 Re-image OS volume OR at minimum: wipe %TEMP%, C:\ProgramData cache, patch fully, re-join to CLEAN domain controller (do NOT re-connect while other nodes still infected).

3. File Decryption & Recovery

Current verdict (May-2024): NO KNOWN PUBLIC DECRYPTOR – the threat actors use:

  • ChaCha20 for file data (256-bit key, per-machine random).
  • RSA-2040 public key (embedded) to wrap that ChaCha20 key → stored in capi.dat & sent to C2.

Recovery avenues:
a) Personal backups (offline, immutable object-lock, or tape).
b) Windows Volume ShadowCopy – EMP runs vssadmin delete shadows /all but sometimes overlooks: 

- Alternative shadow storage via WMI (check `gwmi win32_shadowcopy`).  
- Solution: ShadowExplorer, or `cmd → vssadmin list shadows` → copy intact versions.

c) File-server snapshots / cloud point-in-time (OneDrive “Files Restore”, Azure blob snapshot, AWS S3 versioning).
d) Professional negotiators / data-recovery firms (no general decryptor, yet they may broker for lower demand – currently 1.2 BTC flat, occasionally 0.6 BTC if paid < 72 h).

DO NOT upload the ransom note e-mail to random “decryptor” sites – current lure sites spread RedLine stealer.

4. Other Critical Information

  • Extension clash: “.emp” was also used by a defunct 2016 ‘Emperot’ worm – do NOT trust decade-old decryptors.
  • Double-extortion: data posted on “EmpireLeaks” TOR blog if ransom unpaid within 7 d (proof screenshots – 3-12 GB per victim, mostly finance & legal).
  • Decent OPSEC by actors: binaries compiled 1-2 d pre-campaign, always unique mutex Global\emp-{computername}, strings obfuscated with Eziri, C2 over HTTPS with Let’s-E cert – low static footprint, hence 12/71 VT on day-0.
  • No wiper functions observed (files are still encrypted, not overwritten) – therefore shut-down fast usually saves 30-60 % of data if you react within first 5 min of on-screen ransom note.

Key take-away: EMP’s infection chain is preventable with 2024-standard hygiene (e-mail ISO stripping, Exchange SU, MFA-RDP, ASR rules). Once encryption finishes you are facing offline ChaCha20 = backups or negotiation – no free decryptor exists at time of writing. Prepare today; restore tomorrow.