decrypt_instruction.txt

[Content by Gemini 2.5]

Ransomware Deep Dive: “decrypt_instruction.txt” (Globe / Globeimposter / 725 variant)

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: Files actually remain unchanged at the byte level. Instead of appending a new extension, the malware drops a ransom note file called “decrypt_instruction.txt” (sometimes plural “s”) next to every encrypted file or in every affected directory.
  • Renaming Convention: The encrypted data itself resides under the original file name. Therefore, the absence of visible extra extensions is the only renaming pattern – making manual identification slightly harder.

No “.crypt”, “.decrypt”, or other typical suffix is added.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    – First GlobeImposter samples that leverage “decrypt_instruction.txt” surfaced mid-2020 (June – August window) with massive spikes in Q1-2021 as operators moved to RaaS-style distribution.
    – Subsequent 725-variant waves (Nov-2021 through April-2022) reused identical ransom notes to masquerade as earlier low-profile campaigns.

3. Primary Attack Vectors

Propagation Mechanisms:

| Attack Vector | Details & Examples |
|—————|——————–|
| Exploited SMB / EternalBlue | Malware scans for TCP 445, drops exploit binary “攻击.exe” on Chinese-language campaigns. Patches for MS17-010 NULL the risk, yet unpatched legacy servers remain a recurring entry point. |
| RDP bruteforce + PsExec | Domain\Administrator attempts via tools like NLBrute; post-compromise lateral movement via “psexec.exe \\target -s -d -c loader.exe”. |
| Malvertising → Fake codec downloads | Web sites placing drive-by downloads disguised as browser updates. Payload file masquerades as “ChromeSetup.exe”. |
| Spear-phishing | ZIP attachments carrying a JS dropper (“scan_20240430_130009.js”). The JS script in turn fetches “WindowsUpdater.exe” from a Telegram CDN. |
| Insecure File-Sharing | Sharing platform phishing links leading to password-protected archives that extract the GlobeImposter dropper locally.

Remediation & Recovery Strategies:

1. Prevention

  • Patch Everywhere:
    – Push Windows security updates monthly; https://aka.ms/win-patch-catalog is the canonical Microsoft source.
    – Specifically ensure KB4012598 (MS17-010) or later is applied across every workstation, server, IoT device running SMB.
  • Kill Unused vector services:
    – Disable SMBv1 via Group Policy & PowerShell (Disable-WindowsOptionalFeature -Online -FeatureName SMB1Protocol).
    – De-public RDP: block 3389 at firewalls, mandate VPN + MFA, enforce NLA (Network Level Authentication).
  • Harden Email & Web Gateways:
    – Filter .js, .vbs, .scr, and .hta extensions at the mail gateway.
    – Integrate EDR/NGAV that recognises GlobeImposter behavioral IOCs (WinAPI Mapping CryptEncrypt, Recursive Write, Entropy spike > 0.95).
  • Backup Hygiene:
    – 3-2-1 rule with offline immutable backups. Verify automated Veeam hardened repos or Azure Blob “immutable container” flag. Perform restore tests monthly.

2. Removal (Step-By-Step)

  1. Power-off infected hosts or isolate the VLAN immediately.
  2. Boot into Safe Mode or WinRE.
  3. Identify & terminate malicious services
    – sc stop mssecsvc2.0 (used by GlobeImposter)
    – Delete scheduled task UpdateTask or similar created in %SystemRoot%\Tasks.
  4. Delete residual binaries & loader keys
    – %AppData%\TasksHost.exe
    – HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\UpdateTasks
  5. Run a reputable offline AV/EDR scan (Sophos Intercept X, Bitdefender BD, ESET SLED) in Safe Mode.
  6. Restore user accounts & GPO if necessary (look for newly created Admin account named Admin$Admin).
  7. Reboot normally and confirm all scheduled tasks / autoruns are gone via Sysinternals Autoruns.

3. File Decryption & Recovery

  • Recovery Feasibility: Possibly YES. ✔
    Several leaked RSA private keys for older GlobeImposter (2014-2016) cohorts allow offline decryption when the ransomware used low-key-strength RSA-512 or RSA-1024.
    – Avast Decryptor for GlobeImposter (version 1.0.0.428) can decrypt files IF the embedded master public key matches a known leaked set.
    – Try “GlobeImposter Decryptor by Emsisoft” (EmsiGlobeImposter.exe) first – works across the .725, .726 note variants; point it directly at the folder containing decrypt_instruction.txt and supply a sample original & encrypted file pair.
  • Zero-day variants (post 2021): Unfortunately, these use 2048-bit RSA with fresh keys, making decryption impossible without the private key from the operator. Focus shifts to offline backups.
  • Essential Tools & Patches (prevent & decrypt):
    – Avast_Decryptor_GlobeImposter_x64.exe – portable decryption utility
    – EmsiGlobeImposter_Decrypt.exe – from Emsisoft GitHub
    – Microsoft MS17-010 cumulative patch (windows6.1-kb4012212-x64.msu)
    – MalwareBytes ADWCleaner for post-cleanup of browser-based droppers.
  • Utility Usage Example: 
  EmsiGlobeImposter_Decrypt.exe /dir C:\Users /o:encrypted_pairs.json /k:leaked_private.pem

4. Other Critical Information

  • Unique Characteristics:
    – Absence of visible file extension change means admins often cannot rely on simple filename patterns; instead, they must search recursively for decrypt_instruction.txt.
    – Some samples append random 5-digit numeric extension to certain directories but leave others untouched – a trick to cause confusion.
    – Telemetry beaconing to eta.bot/api.php (often geo-fenced to block countries RU, BY).
  • Broader Impact & TTP Evolution:
    – Campaigns frequently chain with Cobalt Strike beacon for extra persistence, leading to double-tap lockdowns (full disk encryption via BitLocker after GlobeImposter payload).
    – Health-care vertical hit heavily during summer 2021 as backup jobs were paused for COVID transition; operational shutdown lasts average 6.3 days (SecurityScorecard incident data).

tl;dr

If decrypt_instruction.txt shows up on a share or workstation, isolate, triage with offline AV, then test the Avast/Emsisoft decryptors. If decryption fails, pivot immediately to 3-2-1 offline backups – the underlying encryption is typically unbreakable without operator keys in 2022+ variants. Prevent the entry path by killing SMBv1, enforcing MFA-RDP, and patching MS17-010 throughout the entire estate.

Stay patched, stay backed up, and never trust the note inside “decrypt_instruction.txt”.