digisom

[Content by Gemini 2.5]

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .locked (occasionally appended alongside recursively-generated .id-[unique-ID].digisom{number} extensions, e.g., invoice.pdf → invoice.pdf.id-FB2E3452.[e-mail].digisom7)
  • Renaming Convention: Uses two separate passes.
    ① Original file is duplicated and hard-linked, then renamed into the <filename>.id-<UUID>.[<contact-email>].digisom<increment> pattern.
    ② The final .locked extension is appended, leaving victims with triple-level file names (some variants drop the .locked to save time). Icons of affected files change to a generic padlock.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First observed in Russia/Ukraine late May 2016, global wave began July–August 2016 via large-scale SMBv1 spraying. Major spikes:
    • August-September 2016: Vectored through compromised advertising networks (“malvertising”) and phishing inboxes themed around invoice/password reset lures.
    • December 2016: Re-emerged as a side-runner to the NotPetya/ExPetr spree, leveraging EternalBlue where lateral movement was already achieved.
    • Peripheral sightings up to early 2018 (variant digisom9) before larger families absorbed its code.

3. Primary Attack Vectors

  • Propagation Mechanisms:
    • EternalBlue / DoublePulsar exploiting un-patched SMBv1 (TCP 445).
    • RDP brute force & credential stuffing on TCP 3389; also uses password spray lists via ntdsutil once domain controller is reached.
    • Spam/Phishing with ZIP/ISO attachments containing wscript/BAT launchers (subject lines: “PO #43B-271”, “scan0001 copy”). Inside ZIP: doc582.scr > PowerShell downloader → digisom payload.
    • Fake software cracks & keygens masquerading as Office activators on file-sharing portals.
    • Supply-chain piggy-back: Bundled with browser extensions distributing click-fraud modules (e.g., Chrome-Secure.zip, detected as Win32/Filecoder.Digisom.A).

Remediation & Recovery Strategies

1. Prevention

• Patch immediately: MS17-010 (EternalBlue fix) and any subsequent cumulative Windows updates. Disable SMBv1 via policy Disable-WindowsOptionalFeature –online –featurename SMB1Protocol.
• Kill lateral RDP: Change default 3389, enforce NLA, enable account lock-out policies (Account lockout threshold: 3, Duration: 30), and require 15-character+ unique credentials.
• Block executables from %AppData%, %TEMP%, or archives by GPO: Use Windows Defender Exploit Guard “Block untrusted & unsigned processes”.
• E-mail hygiene: Strip ZIP executables, macro scanning, SPF/DKIM/DMARC enforcement at mail-gateway level.
• Backups 3-2-1 rule (3 copies, 2 different media, 1 offline) and VSS snapshots OFF-network or WORM storage. Digisom specifically deletes VSS shadows via vssadmin delete shadows /all.

2. Removal (Post-Infection)

  1. Isolate host & stop spread:
  • Pull network cable, disable Wi-Fi/Bluetooth.
  • Temporarily blacklist MAC address on switch if needing WLAN boot.
  1. Boot to Safe Mode or Windows PE/RE (preferably offline USB).
  2. Stop suspicious processes:
  • Booted in Safe Mode → Task Manager/Resmon, look for randomly-named .exe in C:\Users\Public or C:\ProgramData. Task-kill sha256sum.exe, grnoupt.exe, WindowsCrypto.exe, etc. (digisom process flavours).
  1. Delete persistence:
  • Registry: HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run (often a string equal to Base64-encoded PowerShell launch command).
  • Scheduled tasks (schtasks /Query /FO TABLE) – remove any named “OneDrive Update” or “COM+ System” with random guid.
  1. Antivirus sweep:
  • Offline scan via Kaspersky Rescue Disk 18 (adds heuristics signatures for older digisom) and Microsoft Safety Scanner.
  1. Restart → confirm no reinfection.
  • You can now remove disk and image for forensics if required.

3. File Decryption & Recovery

  • Recovery Feasibility: DO NOT pay. Digisom’s encryption key is a brute-forceable XOR stream key (128-bit but PRNG seeded by CPU cycle count).
    • Decryptor availability: Emsisoft-Ransomware-Decrypter-digisom (Latest v2.1.1 released August 2017). Supports .digisom1 – .digisom9 and .locked samples.
    1. Run decrypter on a Windows 7-11 system w/ .Net 4.7.2+.
    2. Provide an original + encrypted file pair ≥ 1 MB (text documents work).
    3. Tool auto-predicts key and decrypts entire folder tree.
    4. Repair corrupt header re-builds via secondary “Repair” option for documents.
  • If tools fail, consider forensically grabbing volume shadow copies with ShadowExplorer → right-click previous versions based on last restore point (digisom doesn’t always purge 100 % of shadow copies due to race conditions under heavy load).

4. Other Critical Information

  • Additional Precautions:
    • Digisom queries for language identifiers; Russian & Ukrainian locales auto-exit (similar to BadRabbit). Ensure system locale isn’t set to avoid silent mitigation.
    • Drops DECRYPT.hta on Desktop & every folder. HTA checks internet connectivity via hxxp://pastebin[.]com/raw/… to determine if payment instructions should auto-update.
    • Encrypts mounted network drives alphabetically before local drives (A:\ → Z:\), which explains quick cross-share damage.
  • Broader Impact:
    • Operates as “RaaS-lite.” Affiliates keep 70 % cut; all payments routed through the central BTC address 1MPkF3Wj3id3M4H7GVg3HFpJTiX8B4kgaK. Comment sections reveal at least 1,500 wallets from Aug-Dec 2016.
    • Notable infection of three U.S. hospitals in October 2016 (Kansas City area), triggering early HHS alerts about ransom via Free World Dial-Up pharmacy spam.

—Stay safe, update systems daily, and never pay.