decrypt_instructions.txt

[Content by Gemini 2.5]

Technical Breakdown: decrypt_instructions.txt ransomware / STOP(Djvu) Variant

(STOP represents the underlying malware family; “.decrypt_instructions.txt” is simply the ransom-note name that STOP places in every folder after encryption.)

1. File Extension & Renaming Patterns

• Confirmed File Extensions Used by the Active STOP Strains
– .mpal, .qepi, .lezp, .sqpc, .koti, .pola, .coharos, .npsk, .gero, .nosu, .vusad, and dozens of 4-letter variations reported almost weekly.
– A single STOP incident typically displays only one of these extensions (chosen at random), but every folder will contain decrypt_instructions.txt.
• Renaming Convention
Original: report.xlsx
After encryption: report.xlsx.koti (new file extension appended, original extension kept).
The file name itself is never scrambled, so the .koti (or other) tag at the end is the only visible change.

2. Detection & Outbreak Timeline

• First STOP Sample: Late-December 2017 as “STOP-Djvu” (Trojan:Win32/Spade).
• Modern Extension Waves:
– Jan 2022 – present the project forked into what researchers call “NewSTOP”; variants like .mpal and .qepi emerged in March 2023–April 2024 with over 350 new builds uploaded daily.
• Global Activity Peaks: Friday-morning email bursts and fake-software crack torrents typically precede large user reports each weekend.

3. Primary Attack Vectors

  1. Pirated Software & “Crack” Installers – The dominant channel today. Fake activation tools (KMS, Adobe, Office keygens) bundle the payload inside the wrapping installer.
  2. Malicious Email Attachments – Office macro docs (.docm, .xlsm) named like invoice_2024-04-04.xlsm, usually geofenced to specific regions.
  3. Drive-by Torrents & Malvertising – Torrent pages execute malicious JavaScript that silently drops the loader.
  4. Remote Desktop & Brute-forced SMB (rarer now): older builds exploited EternalBlue and RDP credential spraying; modern strains almost never use SMB but will still pivot inside networks once started on any endpoint.

Remediation & Recovery Strategies

1. Prevention

• Patch & Harden –
– Ensure the April 2017 MS-17-010 (EternalBlue) patch is present on all Windows versions.
– Disable SMBv1 via PowerShell: Disable-WindowsOptionalFeature -Online -FeatureName SMB1Protocol.
• E-mail Defenses – Block macro-execution from Internet-originated Office macros using Group Policy (VBAWarnings=2, vbadisablem=1).
• Application Control – Turn on Windows Defender Application Guard or roll out AppLocker/Intune policy to prevent unsigned binaries.
• Crack-culture hygiene – Institute a strict software-asset-management policy: issue KMS servers, ban third-party activation tools.
• Credential hygiene – Enforce MFA for RDP access; lock accounts after 5 failed logins.

2. Removal (Step-by-Step)

  1. Physically isolate infected machines – pull network cables/Wi-Fi.
  2. Boot into Safe Mode with Networking.
  3. Download & run an on-demand scanner:
    – ESET Online Scanner or Malwarebytes. Real-time remediation removes:
    – %AppData%\[random]\updatewin.exe (main payload)
    – Scheduled tasks named “Time Trigger Task” or “Core Init Dll”.
  4. Delete malicious registry keys: 
   HKCU\Software\Microsoft\Windows\CurrentVersion\Run\SysHelper
   HKLM\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\Run\SystemProtection
  1. Restore Windows Defender definitions (ransomware often disables them).
    Run: MpCmdRun.exe -SignatureUpdate.
  2. Reboot normally and re-scan to confirm no residual dropped files.

3. File Decryption & Recovery

• Is Decryption Possible?
– Yes — but only for files encrypted by OFFLINE keys (MD5 hash in ransom note ends with t1).
– No — Online keys (distance ending with t2, or no “VK” line at all) remain irreversible (except via real ransom).

• Free Decrypter Tool
– Download Emsisoft STOP-Djvu Decryptor (current as of 09-Dec-2023, v1.0.0.4): https://go.emsisoft.com/vp/stop-decryptor
– Usage:

  1. Place both PersonalID.txt (inside _readme.txt) and an original, unaltered copy of one encrypted-plus-original file pair on your Desktop.
  2. Launch the tool, select one file pair → Start.
  3. If it reports “This file is encrypted by an ONLINE key”, recovery via decryptor is impossible.

• No Backups → Partially Recover
– Use PhotoRec/TestDisk or Recuva to salvage shadow copies (STOP deletes VSS quickly but sometimes misses USB or long-path volumes).
– For databases, use Oracle MySQL .ibd file repair scripts or SQL Server BAK/MDF recovery if backups exist outside infected volumes.

4. Other Critical Information

• Ransom Note Content (decrypt_instructions.txt) always contains identical text with two e-mails under firemail.de / outlook.com given in the generic template: 

  e-mail: [email protected]
  alternative e-mail: [email protected]
  ID: [upper-case 36-char string]

The extension in file names (e.g., .koti) is not mentioned – that is a deliberate OPSEC trick to force victims to send the PersonalID.

• Potential Data Leakage?
STOP ransoms rarely exfiltrate; no Maze/Conti-style “we stole your documents” paragraph. However, infected machines often host AZORult or RaccoonStealer, causing a secondary breach. Defenders must still assume credentials are in hand.

• Impact Recognition & Colloquial Labels
– Reddit/Forums label the campaign generically as ”DJVU”; do not look for a stand-alone “decrypt_instructions.txt family” in threat intel feeds—it is merely STOP.
– Telemetry shows highest hits in Brazil, India, Turkey, and secondary markets in EU where Office piracy is endemic.

Executive Take-away
If you recover from STOP/Djvu once, invest immediately in Microsoft 365 backup (OneDrive rewind 560 days) plus 3-2-1 off-site immutable backups. Every new STOP wave involves trivial IOCs that AV engines detect within hours of samples hitting VMware; fast patching and licensing hygiene stop re-infection cold.