decrypt_readme.txt

[Content by Gemini 2.5]

Ransomware Variant Analysis
Extension Associated: NOT via an extension appended to files – victims first notice filenames such as “decryptreadme.txt”, “!Decrypt-All-Files.txt”, “readmeto_restore.txt”, etc. Because these are the ransom notes, it strongly signals infection by the Dharma/CrySiS ransomware family.

Technical Breakdown:

1. File Extension & Renaming Patterns

| Attribute | Details |
|—|—|
| Extension(s) actually added to encrypted files | .{original filename}.{ unique 6–12 char ID or email }.{dharma-style extension}
(e.g., report.docx.id-9A3E7BC1.[[email protected]].arrow, invoice.xlsx.id-1A3B5C.[[email protected]].harma) |
| Typical ransom-note filenames | decrypt_readme.txt, readme_to_restore.txt, Info.hta (popped inside every folder), RETURN FILES.txt, or FILES ENCRYPTED.txt |
| Note Screen Lock Behaviour | No full-screen lock; it often just places the HTA popup at logon. |

2. Detection & Outbreak Timeline

  • First public sightings: Early 2016 (CrySiS core code), later rebranded/hacked and sold as Dharma.
  • Rapid diffusion periods: Q3–Q4 2017 (RDP brute-force wave), resurfaced mid-2019, and again 2021–2023 after source code leaks on underground forums.

3. Primary Attack Vectors

  1. RDP / External Services – brute-forcing weak credentials, scanning 3389/445 externally.
  2. Phishing with ZIP-ed JavaScript droppers (.js, .wsf, .hta) that fetch the ransomware.
  3. Exploit Kits (legacy) – older infections still via Fallout or RIG pushing outdated Flash/Java.
  4. Patch-less, self-propagation – unlike WannaCry it does not exploit EternalBlue/SMB v1, but once inside it:
  • Dismantles PowerShell logging, deletes shadow copies (vssadmin delete shadows /all).
  • Spreads laterally through accessible SMB shares, PSEXEC, WMI or PSExec.EXE kits manually prepared by the intruder.

Remediation & Recovery Strategies:

1. Prevention

  • Lock remote access:
    • Disable RDP on the internet-facing adapter, or enforce VPN-tunnel only, MFA, and IP whitelisting.
    • Enforce complex passwords + minimum length 12, lockout 5/30/60 thresholds.
  • System hygiene patches:
    • Windows KBs: Prioritise all Windows cumulative updates 2017 onwards (none essential for this strain, but good practice).
    • Disable SMBv1 (via Disable-WindowsOptionalFeature -online -FeatureName smb1protocol).
  • Network segmentation & EDR:
    • Deploy behaviour-based EDR (CrowdStrike Falcon, Microsoft Defender + ASR rules).
    • Create firewall allow-lists between VLANs that deny 445/RDP in lateral hops.

2. Removal

Step-by-step (offline):

  1. Disconnect from network – pull LAN/Wi-Fi to halt propagations.
  2. Boot into Safe Mode with Networking (or Safe Mode offline).
  3. Remove persistence:
    Registry startup → HKCU\Software\Microsoft\Windows\CurrentVersion\Run and HKLM\…Run → delete any *.exe unknown payloads.
    Scheduled Tasks → check “Updater”, “MsUpdate”; delete malicious tasks.
  4. Run full Anti-malware scan: Malwarebytes, ESET, Bitdefender, or Defender offline scan → everything under %APPDATA%\{random 4-8 chars}.exe typically goes away.
  5. Advanced cleanup: If the post-explo tool Mimikatz, ChromePass, or LaZagne was dropped, manually remove those traces as well.

3. File Decryption & Recovery

  • Feasibility: As of 2024 NO free private-key brute-force or decryption tool exists for current Dharma/CrySiS variants.
  • Historical Exceptions:
    – August 2021 patch / June 2016 master keys do NOT cover the latest wave that produced these decrypt_readme.txt notes.
  • Your options:
    A. Check shadow copies: vssadmin list shadows. If not wiped, mount shadow volume and copy out unaffected versions.
    B. Use ShadowExplorer, Recuva, PC Hunter for residual files.
    C. Bare-metal restore from recent offline immutable backup (Veeam, Commvault, Synology Hyper Backup w/locking, Windows Server Backup to non-domain account).
    D. File-carving: If data deletion was via overwrite vs. quick format, tools like PhotoRec may pull certain Office documents out of NTFS slack.

Snapshot Guide (one-liner PowerShell backup test): 

wbadmin start backup -backupTarget:\\NAS\daily_backups_$(Get-Date -F yyyy-MM-dd) -include:C: -allCritical -vssFull -quiet

Test restore weekly and rotate destination offline weekly.

4. Other Critical Information

  • Malware behaviour quirks: Dharma executables have invalid digital signatures or self-signed certs; often hashes change every drop to evade AV signatures.
  • Ransom demand notes tag an email (e.g., [email protected]) and include ID + BTC wallet, but payment does not guarantee decryptor – common secondary extortion (data leaking) follows.
  • Notable campaigns:
    – 2020–2021 hospital attacks (U.S., Brazil), server farms holding EHR backups hostage.
  • Long-term impact: After removal, search entire domain for stale RDP/service accounts and perform cleanup of all credentials stored in plain text (KeePass roll-out recommended).

Bottom line: Dharma/CrySiS remains very active in 2024. While decryption is currently impossible, robust offline backups plus hardened remote-access posture are the proven, battle-tested defense.