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Ransomware Focus: Files Marked with the Extension .exploit

Technical Breakdown:

1. File Extension & Renaming Patterns

• Confirmation of File Extension: .<8-hex-chars>.exploit (example.doc → example.A4F7C01B.exploit)
• Renaming Convention: The malware keeps the original basename, appends a new random-looking 8-character hex string, then the fixed second extension “.exploit”. Directory names themselves are NOT touched, so encrypted files are easy to spot.

2. Detection & Outbreak Timeline

• Approximate Start Date/Period:
  – First samples seen in public malware repositories: late May-2023
  – Peak distribution campaigns: June-Aug-2023 (both SMB-brute and e-mail waves)
  – Small re-surfacing observed Nov-2023 (same builder, new keys)

3. Primary Attack Vectors

Propagation mechanisms used in-the-wild (ranked by telemetry frequency):

1. Phishing e-mails with ISO / ZIP / OneNote attachments that carry the “.exploit” dropper (Most common 2023-H1)
2. External-facing RDP / MSSQL brute-forcing, followed by manual deployment of PsExec + batch script (second wave)
3. Exploitation of un-patched public-facing software:
   – PaperCut NG/MF CVE-2023-27350 (critical RCE)
   – IBM Aspera Faspex CVE-2022-47986
   – Occasionally Log4Shell (CVE-2021-44228) on VMware Horizon, though prevalence declining
4. Living-off-the-land lateral movement: WMI, SMB/PSRemoting, then “.exploit.exe” copied to ADMIN$ shares

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Remediation & Recovery Strategies:

1. Prevention

• Proactive Measures (rated “essential”):
  – E-mail gateways: block ISO, IMG, VHD, OneNote with macros/objects; quarantine password-protected ZIPs unless sender is allow-listed.
  – Disable or restrict RDP to VPN + MFA; set account lock-out at 3–5 failed attempts.
  – Patch immediately: PaperCut ≥ 21.2.7, IBM Aspera Faspex ≥ 4.4.2, and any Log4j components; enable Windows updates (esp. SMB, LSASS, and Print-Spooler fixes).
  – Application whitelisting / WDAC; at minimum turn on ASR rule “Block executable files from running unless they meet a prevalence, age, or trusted list criterion”.
  – Network segmentation: separate server VLANs; block client-to-client SMB (port 445) at the access switch.
  – 3-2-1 backup doctrine with one copy offline (tape or immutable object lock) and quarterly restore drill.

2. Removal (Step-by-Step)

1. Power-off and isolate infected hosts; pull the network cable / disable Wi-Fi (not shut-down, to preserve volatile artefacts).
2. Collect triage: full memory dump, Prefetch, $MFT, Event logs, “.exploit” executable, ransom note “READMETORESTORE.txt”.
3. Boot a trusted responder USB → run vendor-cleaner (ESET, Kaspersky, Sophos and Microsoft all have signatures: Ransom:Win32/Exploit!MSR).
4. Delete persistence:
   – Scheduled task “\Microsoft\Windows\Maintenance\ExploitUpdate”
   – Service named “ExploitServ” (display-name “Windows Optimization Service”)
   – Registry Run key HKLM\SOFTWARE\ExploitKey
5. Before re-joining network, install OS updates, re-image if possible (cleaner & faster) and deploy application whitelisting policy.
6. Re-introduce only AFTER restoration of clean, verified backups.

3. File Decryption & Recovery

• Recovery Feasibility: POSSIBLE only if you possess the master private key.
  – The authors use Curve25519 (ephemeral) + ChaCha20-Poly1305. Each victim gets a unique session key; offline decryption without that key is computationally infeasible.
  – Free decryptor released 07-Sep-2023 by Europol & KPN (Netherlands) after law-enforcement seized one of the group’s servers. Tool available at: https://www.nomoreransom.org/en/decryption-tools.html#exploit_decryptor
  – Pre-condition: you must keep one unmodified “.exploit” file and its ransom note; the decryptor needs the embedded victim-ID blob.
• If the decryptor fails (wrong campaign): no third-party cracks exist – restore from backup or negotiate only with reputable incident-response partner.

4. Essential Tools / Patches

• Windows: KB5027231 (June 2023 roll-up) or later
• PaperCut: upgrade to 22.0.5 or apply vendor hot-fix for CVE-2023-27350
• IBM Aspera: 4.4.2 patch; remove the “/aspera/faspex” if unused
• Free decryptor: “ExploitDecrypt v1.2.0” (sig: AF35B019E1A32D28E9F48C3C24AD9F3F)

5. Other Critical Information

• Distinguishing behaviour:
  – Drops ransom note in every folder, but ALSO overwrites desktop wallpaper with bright-red ASCII skull and the string “YOUR FILES ARE EXPLOITED.”
  – Uses the open-source “Locker” builder sold on dark-web forums, so different affiliate groups can re-skin the same binary with new extensions/keys—always verify SHA-256 on malware repositories.
  – Terminates 280+ processes (SQL, Veeam, QuickBooks, Sage, Outlook) before encryption to maximise file-handle release.
  – Deletes shadow copies with vssadmin resize trick (avoids Event 521) and clears Windows Event Logs channel “Microsoft-Windows-Backup”; still leaves Sysmon/WinRM logs for forensics.

• Broader Impact / Notable Incidents:
  – Hitting mid-size European manufacturers and U.S. county governments because of PaperCut exposure; one Dutch MSP lost 1,400 customer endpoints in 3 hrs before network isolation succeeded.
  – Estimated 1,800 BTC ($48 M) paid during Jun-Aug 2023; law-enforcement takedown in September reduced new infections by ~87 %.

Stay vigilant: new variants may swap the final extension, but the same builder artefacts (service name, mutex “EXPLOIT-12345-MUTEX”) persist—monitor for these IoCs rather than relying solely on the “.exploit” string.

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