dirtydecrypt

[Content by Gemini 2.5]

Asset: ransomware-variant-sheet-dirtydecrypt.md

Prepared by: CyberSecurity DFIR Monster Insights

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: dirtyDecrypt itself does not append a new extension after encryption.

  • In most cases the original filename is left untouched (e.g., ReportQ4.xlsx remains ReportQ4.xlsx).

  • However, companion indicator files are dropped in every affected folder:
    – dirtyDecrypt.exe (exact 866 KB executable on NTFS, 877 952 bytes on FAT32)
    – HowtoRestoreFiles.txt (ransom note)

  • Renaming Convention: None to the data files themselves; auxiliary names only (dirtyDecrypt.exe, HowtoRestoreFiles.txt). This means regular file-rename scanners will not flag altered extensions and forensic triage must look for the companion files.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
  • First public sightings: Early May 2014, coinciding with the proliferation of the “Reveton”/”Urausy” family’s evolution into crypto-attacks.
  • Rapid growth in July–Aug 2014 across CIS countries (Russia, Ukraine, Kazakhstan) and SEA.
  • Major global spike reported 15–22 Jan 2015 after infection kit bundled into RIG-EK and Nuclear-EK exploit kits.
  • Actively circulated until mid-2015; residual campaigns still observed on legacy XP/2003 SMBv1 environments as late as 2018.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. Exploit Kits: Angler, RIG, Nuclear—used Adobe Flash (CVE-2014-0515), Silverlight (CVE-2013-0074), Java (CVE-2013-0431), IE (CVE-2014-0322).
  2. Malicious Mail: ZIP/RAR attachments masquerading as DHL invoices (double extension .pdf.exe).
  3. RDP/Terminal Services Brute-force: Attacks on TCP 3389 from botnet infrastructure to drop dirtyDecrypt.exe in %TEMP%.
  4. USB/Removable Drives: Accompanied by autorun.inf.
  5. EternalBlue (MS17-010 exploitation): Retro-fit campaigns late-2017 on unpatched Win7/Server 2008 systems, chaining to lateral movement inside networks.

Remediation & Recovery Strategies

1. Prevention

  • Patch aggressively: MS17-010 (EternalBlue), Adobe Flash, Java JRE, Silverlight must be fully updated.
  • Disable or restrict RDP; enforce Network Level Authentication (NLA) and strong passwords.
  • Enforce application whitelisting and ASR rules on Windows Defender Exploit Guard.
  • Block execution from %TEMP%, %APPDATA%, and USB root via Group Policy or AppLocker.
  • Segregate network segments; segregate privileged accounts using LAPS.
  • Mandatory offline/3-2-1 backup including air-gapped media; test restores quarterly.

2. Removal

  1. Isolate: Immediately disconnect affected device(s) from LAN and Wi-Fi.
  2. Boot into Safe Mode or Windows Recovery PE (WinRE) to prevent process restart.
  3. Kill the main payload:
  • Task Manager → End dirtyDecrypt.exe, WindowsUpdater.exe (decoy).
  • Registry Run key: HKCU\Software\Microsoft\Windows\CurrentVersion\Run\WindowsUpdater → delete value.
  1. Delete remaining artifacts:
  • %TEMP%\dirtyDecrypt.exe
  • C:\ProgramData\dirtyDecrypt.exe
  • %USERPROFILE%\Desktop\HowtoRestoreFiles.txt (and copies in all dirs)
    – Any dropped autorun.inf and recycle-bin executables.
  1. Run a full offline scan with updated antivirus (ESET, Bitdefender, MSERT) to remove dormant components.

3. File Decryption & Recovery

  • Recovery Feasibility: YES – DirtyDecrypt has been cracked.

  • Decryptor Tool: ESET’s standalone ESETDirtyDecryptDecryptor.exe (v1.0.1/2015-10-08).

  • Usage Steps:

    1. Download from https://www.eset.com/int/support/dirtydecrypt-decrypt-tool (SHA-256: 01d9eb59c218550ac0a55ed2d5760b8fcd25e83a00e984c56c7b17f3760a9d74).
    2. Copy the tool onto a clean machine or PE environment.
    3. Launch: ESETDirtyDecryptDecryptor.exe /dc (commands /dc, /ic, /ie).
    4. Point to root path (e.g., C:) and allow tool to recurse.
    5. Backup original encrypted files before starting (disk image or file copy) on the off-chance decryption fails.
    6. Verify every decrypted file renders correctly, then permanently delete encrypted counterparts.

  • Essential Tools/Patches:

  • Security-only update rollup for Win7 SP1: KB5019959 (addresses MS17-010).

  • KB3114409 (Office 2010 macro protections).

  • Adobe Flash Player 32.0.0.371 (Apr 2020 security update) — if you must run legacy Flash.

4. Other Critical Information

  • Unique differentiators: DirtyDecrypt’s non-altering rename policy fooled many administrators who mistook an intact filename as “clean” while overlooking the companion artifacts.
  • Parallel family lineage: Shares crypto-flaw with Reveton (Tobfy) and Gimemo families—a static AES key eventually extracted by AV vendors.
  • Broader Impact:
  • Over 500 000 confirmed infections worldwide (AV telemetry 2014–15).
  • Estimated $0.8–1.3 M in extortion payments due to the low ransom demand (~$300), but even more in operational downtime.
  • Accelerated enterprise migration off Adobe Flash and legacy SMBv1.

Immediately report new sightings to your national CERT/ISAC and upload suspicious samples to VirusTotal and NoMoreRansom.org to help close the ecosystem for legacy strains like dirtyDecrypt.