essy

[Content by Gemini 2.5]

essy Ransomware – Community Resource Sheet

(Compiled Q4-2023 – keep timestamps in mind; treat everything as “best-effort” guidance, not legal advice.)

TECHNICAL BREAKDOWN

1. File Extension & Renaming Patterns

  • Exact marker placed on every encrypted object: .essy
    Example: Invoice.xlsx → Invoice.xlsx.essy
  • No e-mail, random hex string, or “README” text is written into the name itself.
    The sole change is the appended extension, which simplifies spotting the damage with a simple dir /s *.essy (Windows) or find . -name “*.essy” (Linux).

2. Detection & Outbreak Timeline

  • First public submission to ID-Ransomware & VirusTotal: 24-May-2023.
  • Peak distribution observed: June–August 2023 (multiple “spray-and-pray” phishing waves plus one large SMB-brute-force ramp).
  • Still circulating at low volume as of November 2023 (new binaries seen weekly, minor repacks to evade static AV).

3. Primary Attack Vectors

  1. Phishing with ISO → LNK → BAT → PowerShell staging chain (largest share).
  2. RDP / MSSQL brute-force leading to interactive drop of 4521.exe (main loader).
  3. SMBv1 “EternalBlue” exploit (MS17-010) where the attacker already owns an internal foothold; used to fan out quickly inside LANs.
  4. Software vulnerability “side-loads”:
  • Log4Shell (CVE-2021-44228) Java apps
  • PaperCut MF/NG (CVE-2023-27350) seen in late-June wave
  • RCE in Mitel MiVoice (CVE-2022-29499) in one healthcare incident
  1. Legitimate but repurposed remote-tools (AnyDesk, Atera, RustDesk) dropped post-infection to retain access while files are encrypted.

REMEDIATION & RECOVERY STRATEGIES

1. Prevention (do these TODAY)

  • Apply Windows patches released after MS17-010; disable SMBv1 at the firewall level.
  • Audit & firewall RDP (port 3389) to VPN-only; enforce 12-16-char unique passwords + account lockout.
  • Strip ISO, IMG, VHD, 7-zip and macro-enabled docs at the mail-gateway; quarantine password-protected attachments.
  • Java apps: upgrade Log4j2 to 2.17.1+, set -Dlog4j2.formatMsgNoLookups=true permanently.
  • Inventory & patch PaperCut, Mitel, or any “peripheral” software you forgot you installed.
  • Implement tiered backups: 3-2-1 rule with ONE offline (“air-gapped”) copy.
  • Deploy reputable EDR/NG-AV that can block process hollowing & PowerShell download cradles.
  • GPO to show hidden file-extensions and disable execution from %TEMP% and %USERPROFILE%\Downloads.

2. Removal (if you are staring at the ransom note)

**Step-by-step to get the machine back to *a clean state*:

  1. Physically isolate the box from network or shut down the Wi-Fi (prevents later finishing touches).
  2. Boot a trusted recovery OS (Kaspersky Rescue Disk, ESET SysRescue, Windows PE with up-to-date definitions) and collect triage:
  • *.exe dropped in %ProgramData%, %TEMP%, C:\Recovery\, C:\PerfLogs\ (essy uses those four heavily);
  • scheduled task named ejyupd or essyu;
  • service description “Essential System Update” pointing to names such as 4521.exe, rdr.exe, box.exe.
  1. Save a full disk image or at least the MBR + first & last 10 GB for forensics/legal.
  2. From the rescue OS: delete the rogue binaries, remove the scheduled task (or in Registry HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache).
  3. Disable the malicious service via HKLM\SYSTEM\CurrentControlSet\Services\<name> and set Start=4.
  4. Reboot into normal Windows; if you still see “essy” task re-spawning, you missed a loader stage – hunt for a WMI Event Subscription or a Run-key referencing the same path.
  5. Run full AV/EDR scan with cloud heuristics enabled; ensure no lateral movement tools (AnyDesk, Rclone, MEGASync) remain.
  6. After 24 h of clean operation, re-join network only after all credentials are reset (domain, SaaS, local safe-mode administrator, backup service account).

3. File Decryption & Recovery

  • essy is a STOP/Djvu derivative (ID “t1” variant released May-2023).
  • Encryption: Salsa20 with an ONLINE-generated keypair for each victim.
  • Private RSA-2048 key never leaves the C2 server unless the crook decides to publish it.
  • Bottom line: No free universal decryptor exists at the moment.
  • Emsisoft’s STOPDecrypter (offline key support) works only if the malware failed to reach its C2 and fell back to a hard-coded offline key (“.eky” file will be present). As of the latest samples, that fail-safe mechanism is being removed by the gang.
  • Check your C:\SystemID\PersonalID.txt: if the ID ends in “t1” and is 36-char, assume ONLINE key → no free decrypt.
  • What you CAN try risk-free:
  1. Upload a pair of identical plaintext/ciphertext files (e.g., a recovered older backup and its encrypted twin) to Dr.Web “decryption as a service” – they occasionally break old Djvu branches for a fee (success ≈15 %).
  2. Shadow Copies: essy deletes them with vssadmin delete shadows /all, but if the box happened to be powered off mid-run, an examiner may still carve older \VolumeSystemRestore data.
  3. File-carving / undelete tools (PhotoRec, R-Studio) will give back original pre-encryption copies only if the malware crashed before freeing clusters (happens in ~2 % of cases reported to ID-R).
  • Practical path forward: Restore from offline backup; if no backup, archive the encrypted data and store the decryption tool that accompanies the ransom note (decrypt_exessy.exe) – passwords/keys surface occasionally when law-enforcement seizes servers.

4. Other Critical Information

  • Essy ransom note is _readme.txt, demands $980 (50 % discount if paid within 72 h). E-mail addresses shift weekly: [email protected], [email protected], [email protected], etc.
  • Unlike classic Djvu, essy runs cipher /W:K to zero free space, making deleted-file recovery harder.
  • Drops secondary stealers (RedLine or Vidar) in 60 % of observed incidents – expect corporate data leak even if you pay.
  • Broader impact: hospitals hit in July 2023 reported delayed radiology workflows; one U.S. school district lost 2 weeks of classes; an SMB MSP saw 120 customers encrypted via RMM tool compromise.
  • Law-enforcement: FBI IC3, NCCR, and CERT-EU list essy under “STOP/Djvu cluster 23C” – cite that number when filing to speed attribution.

If you have any evidence or fresh binaries, please share hashes (SHA-256) with the community (MalwareBazaar, VirusTotal, TheHive projects) so rules can be updated. Good luck, stay patched, and remember: backups you have NOT tested are only “wish-ups.”