eslock

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension:

  • .eslock

  • The extension is appended to the original filename (it does not replace the native extension).

  • Example:
    AnnualReport.xlsx becomes AnnualReport.xlsx.eslock

  • Renaming Convention:

  • No randomised prefix/suffix.

  • No e-mail address or victim-ID in the filename.

  • Directory root is littered with one dropped ransom note named README_DECRYPT.txt (some early waves also dropped HOW_TO_DECRYPT.hta).

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
  • First publicly-reported samples: mid-May 2022 (ID-Ransomware & Twitter spikes).
  • Small-volume campaigns continued through Q3 2022; large uptick in December 2022 after a new affiliate began leveraging e-mail & RDP.
  • Still circulating in 2024, but at lower volume than the bigger players (LockBit, Akira, BlackCat).

3. Primary Attack Vectors

  • Propagation Mechanisms observed in the wild:
  • Exploitation of public-facing services
    • Microsoft Exchange ProxyLogon / ProxyShell (CVE-2021-26855/34473/34523) – common in May ’22 waves.
    • Log4j RCE (CVE-2021-44228) for Linux-hosted application servers found with Wine/CrossOver-installed eslock.
  • Remote Desktop Protocol
    • Brute-forced or previously-stolen credentials (many victims had TCP/3389 open to the Internet).
  • Phishing & malicious e-mail attachments
    • ISO → LNK → BAT → PowerShell dropper chain.
    • Excel 4.0 or VBA macros that fetch the final payload from hxxps://paste[.]ee/r/<random>.
  • Living-off-the-land lateral movement
    • PsExec, WMI, and net use to push the encryptor once an initial foothold is obtained.
  • No evidence of SMB-v1/EternalBlue or self-spreading worm code; infection is human-operated.

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
  1. Patch Exchange and Log4j (all versions) immediately – these are still the #1 enablers.
  2. Block or restrict RDP at the perimeter; enforce MFA for every RDP/VDI session (use RD-Gateway + Azure AD MFA, Duo, or similar).
  3. Upgrade to Windows 10/11 with Windows Defender real-time enabled; enable ASR rule “Block credential stealing from LSASS” and “Block process creations from Office macros”.
  4. Segment networks – VLAN corporate servers away from user subnets; deny SMB/445 between user VLANs.
  5. Maintain 3-2-1 backups: 3 copies, 2 media types, 1 off-line/off-site (tested, immutable, no writable share).
  6. Application whitelisting (WDAC / AppLocker) – eslock is a new unsigned PE, so an effective policy blocks it outright.
  7. E-mail controls: strip ISO, RAR, and macro-enabled Office attachments at the gateway; sandbox everything else.

2. Removal

  • Infection Cleanup (step-by-step):
  1. Physically isolate the machine (pull Ethernet / disable Wi-Fi).
  2. Boot into Safe Mode with Networking or use a clean WinPE/Recovery USB if infra is still down.
  3. Collect a forensic image if legal/insurance require it; otherwise continue with eradication.
  4. Delete the following artifacts (typical paths – adapt to your environment):
    • C:\Users\<user>\AppData\Local\Temp\eslock.exe (main 32-bit payload)
    • %ProgramData%\Microsoft\Windows\esvr.exe (added Run-key)
    • README_DECRYPT.txt in every folder (optional – harmless, but delete for cleanliness)
  5. Remove persistence:
    • HKCU\Software\Microsoft\Windows\CurrentVersion\Run\esvr = esvr.exe
    • Scheduled task EsLocker (under \Microsoft\Windows\Multimedia).
  6. Run a full antivirus scan (Defender or your vendor of choice) – eslock is detected generically as Ransom:Win32/Eslock, Trojan-Ransom.Win32.Encoder, etc.
  7. Patch, reset local admin passwords, and revoke any LDAP/AD accounts suspected to be compromised before returning the asset to the network.

3. File Decryption & Recovery

  • Recovery Feasibility:

  • At the time of writing there is NO public decryptor for .eslock.

  • Encryption method: ChaCha20 for file data, ECDH public key (secp256r1) to wrap the ChaCha session key. Keys are generated per victim and kept only on the attacker side.

  • Free recovery therefore depends entirely on backups or shadow-copies (eslock deletes VSS, but sometimes fails on large repositories).

  • Essential Tools / Patches:

  • eslock_decryptor.exe – does NOT exist (beware of scam sites).

  • ShadowExplorer / vssadmin – worth a try but success rate ≈ 5%.

  • Keep Exchange updated with the latest Cumulative Update + Security Update.

  • The ESET Log4j Vulnerability Scanner or Qualys Log4jScan to find still-unpatched apps.

4. Other Critical Information

  • Additional Precautions / Variant Quirks:

  • Dual payload capability – attackers push a Linux ELF if they land on a VMware ESXi / Ubuntu backup server; same .eslock extension appended.

  • Selective encryption: skips C:\Windows, \Program Files, and \PerfLogs so the OS remains bootable, encouraging victims to pay.

  • Ransom demand: 0.04 – 0.12 BTC (May 2022 USD equivalent ≈ $1 400 – $4 000); note is plain-text, no Tor URL; negotiation e-mail addresses vary per affiliate (often ProtonMail).

  • Double-extortion? No dedicated leak site so far; however, operators exfiltrate data with rclone and threaten to publish via “Data Breach Forums” if unpaid.

  • Broader Impact:

  • Mostly hits SMEs (≤500 seats) in Europe, North America, ANZ.

  • Because it leverages unpatched Exchange and Log4j, many victims were already out of support – illustrating the long-tail risk of “internet-facing legacy”.

  • Usually re-infection occurs within 48 h if owners only restore data and fail to close the original vector (we’ve seen the same exploited Exchange server hit three times in May/June 2022 before proper patching).

Quick-reference cheat-sheet:

  • Extension: .eslock
  • Decryptor? None – rely on backups.
  • Top vectors: Unpatched Exchange, Log4j, exposed RDP, phishing.
  • Must-do fixes: CU+SU for Exchange, disable/arbitrate RDP, kill SMB lateral, test off-line backups.

Share this guide, stay patched, and never pay if you can restore from a clean backup.