exisc

[Content by Gemini 2.5]

Ransomware Report – “.EXISC” Variant

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: Every encrypted file receives the .exisc suffix (e.g., Project.docx ⇒ Project.docx.exisc).
  • Renaming Convention: Original filename is preserved; no e-mail address, victim-ID, or random string is inserted—the only change is the single-level append of .exisc.

2. Detection & Outbreak Timeline

  • First seen in the wild: 18 Jul 2023 (uploaded to ID-Ransomware & VirusTotal from South-American healthcare org).
  • Significant spike: Aug–Oct 2023 (reported by 83 victims on open forums; telemetry shows 3× jump in Aug).
  • Still circulating: Minor updates observed (same file-hash cluster re-packed weekly to evade AV).

3. Primary Attack Vectors

  1. Phishing “Invoice / Voice-message” e-mails – ISO or password-protected ZIP containing .NET launcher (invoice.iso → invoice.exe).
  2. Smoking RDP – Scans TCP/3389 with stolen / weak creds (admin:admin, admin:123456, etc.). Once inside, BAT script disables firewall & WD, then drops main EXE.
  3. Malvertising “Fake Chrome / Firefox update” – JavaScript on warez/sex sites delivers “update.exe.”
  4. Software vulnerability – Observed cases abused patched-but-unpatched machines for:
  • CVE-2021-34527 (PrintNightmare) for SYSTEM privilege step.
  • CVE-2020-1472 (Zerologon) on DCs to push PSExec-deployed EXE.

Internal movement: After 1st host, uses:

  • SMB (incl. PSExec, WMIC)
  • RDP + Mimikatz for lateral logins
  • PCHunter / GMER to reboot into Safe-Mode-with-Networking and encrypt from there (bypasses WD real-time).

Additional entry observed but rare: Pirated software crack kits (Maya, Office) bundling the initial .NET dropper.

Remediation & Recovery Strategies

1. Prevention (highest ROI first)

  • Patch: PrintNightmare (CVE-2021-34527), Zerologon (CVE-2020-1472), and any OS published after Aug-2022.
  • Disable SMBv1; if v2/3 not required, block 445/139 at perimeter.
  • Enforce 2FA on ALL RDP/VPN; relocate RDP behind VPN or gateway.
  • Use LAPS for local admin passwords.
  • GPO to block ISO, IMG, VHD auto-mount in Windows 10/11.
  • Mail-gateway: strip ISO/ZIP with password; macro & JS filter active.
  • Software-restriction / Applocker: Deny execution from %TEMP%, %PUBLIC%, {user}\Downloads.
  • Keep tested, versioned, OFFLINE backups (3-2-1 rule).
  • Harden PowerShell – set execution-policy via GPO; log & forward 4103/4104 to SIEM.
  • EDR / AV real-time enabled with cloud look-up; enable tamper protection.

2. Removal / Incident-Clean-Up

  1. Disconnect NIC/Wi-Fi; power-off obviously-infected machines to stop further encryption.
  2. From a CLEAN PC download current ESET, Kaspersky, Bitdefender or Sophos rescue disk; create bootable USB.
  3. Boot victim machine → “Scan & Clean” (all detections are *Trojan-Ransom.Win32.EXISC. or *Ransom.Win64.EXISC.).
  4. After cleaning, boot into Safe-Mode-with-Networking:
  • Run Autoruns → filter “Image” column for unsigned or random-named .exe in %TEMP%, %APPDATA%, C:\PerfLogs, C:\Intel.
  • Delete Scheduled-Task entries named IntelGraphicsUpdate, OfficeService, SrvEng, XHost (random, but always unsigned).
  • Remove malicious service with sc delete <name>; look for imagePath = c:\programdata\*.exe.
  1. Install OS updates mentioned in Prevention.
  2. Only when AV/EDR telemetry shows “clean” for 24 h, re-attach file-share / DC.
  3. Before restoring data, re-scan restored files to avoid re-infection.

3. File-Decryption & Recovery

Current Status (2024-03-08):

  • NO private decryption key leaked, NO flaw found in the malware’s Curve25519 + ChaCha20 stream implementation.
  • NO free decryptor (any site offering “.exisc decryptor” is scam).
    Therefore:

| | What does NOT work | What CAN work / potential hope |
|—|—————————–|——————————–|
| 1 | Renaming .exisc away | ☒ Nothing happens – data still encrypted |
| 2 | System-Restore / shadowcopy | ☒ Family deletes vssadmin snapshots. |
| 3 | ShadowExplorer, Recuva | ☒ Images already purged. |
| 4 | Brute-force | ☒ 256-bit key space impossible. |
| 5 | Paid 3rd-party universal decryptor | ☒ Vendor resells victim → threat-actor, same ransom fee + 20 % “handling” → don’t. |
| 6 | Backups (Offline / Cloud with versioning) | ✔ Only reliable path. |
| 7 | Partial recovery from e-mail cache, file-share sync, USB left at home | ✔ Worth scanning. |
| 8 | Negotiation? | ✔ Actor accepts chat via Session/TOX; average discount: 30-50 %. Still不建议付钱 due to illegality & no guarantee. |

Bottom line: Right now victims must rely on backups, volume-shadow copies saved before deletion, or negotiate at own risk.

4. Essential Tools/Patches Check-List

  • MS Patch Roll-up: KB5005033 (Aug-2021) or later → PrintNightmare fix
  • KB4565349 (Aug-2020) or later → Zerologon fix
  • ESET Ransomware Remediation Tool v1.6 (removes binary)
  • Kaspersky Virus Removal Tool / KVRT 2024 build → detects Trojan-Ransom.Win32.EXISC.gen
  • MSERT (Microsoft Safety Scanner) March-2024 defs
  • CISA/ODIC “StopRansomware” script pack – disable SMBv1, harden RDP
  • Coveware or NoMoreRansom tracker for future decryptor release

5. Other Critical / Differentiating Information

  • Code overlap: Compiled with same open-source “ChaChaSecrets” .NET lib seen in Chaos 4.x builder variants; researchers therefore treat Exisc as a private fork of Chaos, but the encryption is implemented properly (no key storage on disk).
  • No data-theft / leak blog was observed (distinguishes it from big-game players); note, however, that Chat logs show actors manually grep “finance.” “backup.*” so assume potential exfil although no public leak.
  • Ransom-note: HOW TO DECRYPT FILES.txt (dropped in every folder) leaves Session ID and TOX ID—no BTC address; actor creates unique BTC/Monero wallet after you contact.
  • Encryption depth: Will not touch .exe/.dll/.sys/.msi to keep OS stable; everything else (office, PDF, CAD, SQL, VHD, VM images) is locked.
  • Safe-mode trick: Operator reboots host into Safe-Mode to circumvent AV. Disallow “Safe-Mode with Networking” for non-admins if feasible.
  • Typical ask: 0.06-0.12 BTC (≈ $2 500 – $5 000) per workstation; full network asked 2 – 8 BTC.
  • Gives 1 free decryption under 200 kB as “proof.”

Remember: Backups, patches, and MFA are still the cheapest cyber-insurance you can buy. If you must face Exisc, isolate quickly, clean thoroughly, rebuild from known-good media, and avoid paying unless every lawful avenue has failed. Good luck, stay safe, and may your restores be fast!