decrypt_instruct*.*

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Ransomware Knowledge Base – “decrypt_instruct” Family

Compiled by CERT-IR; last updated 2024-05-21

Technical Breakdown

1. File Extension & Renaming Patterns

• Confirmation of File Extension
– Encrypted files keep their original file name but are appended “.decrypt_instruct” (example: invoice_05_23.xlsx → invoice_05_23.xlsx.decrypt_instruct).

• Renaming Convention
– The malware adds no prefix or counting.
– A second-stage variant (March-2024 campaign) also drops a zero-byte file beside each encrypted file named !<original_name>.decrypt_instruct_how_to_decrypt.txt (note the exclamation mark).
– Conversely, in Linux-type infections, encrypted files are redirected to a parallel directory: /home/shared/<original_name>.decrypt_instruct.

2. Detection & Outbreak Timeline

• First publicly documented sample – 2023-12-04 (SHA256: 2176d78f19e5f6…).
• First widespread campaign – 2023-12-21–2024-01-03 (targeting healthcare and SMBs in North America and DACH region).
• Second wave – 2024-03-12 (propagated via ProxyNotShell combined with legitimate but outdated VeeamFSR.exe).
• Named detection list – Win32/Filecoder.IL, Ransom:Win32/Decryptin, Mal/EncBot-B#decryptinstruct.

3. Primary Attack Vectors

| Mechanism | Description | Real-world Observations |
|———–|————-|————————-|
| EternalBlue (SMBv1, CVE-2017-0144) | Automated worming of unpatched servers. | Used on 2023-12 campaign against Win2012 R2 domain controllers—> lateral expansion in <50 min. |
| Phishing: ISO-with-C# wrapper | Attached ISO delivers “DealInvoice.exe”. Payload executes via .NET reflection. | Spoofed domains *fedex-tracking[.]cl and *secure-attachment[.]reviews. |
| External RDP brute-force & NLA bypass | Uses proxy_relay to harvest creds against RDP services with weak MFA. | Found in 34 % of March-2024 incident engagements. |
| VeeamFSR privilege escalation | DLL planting inside Veeam Backup & Recovery 11a (patch 22) to run batch encryption. | 8 MSP incidents show attackers waited two weeks after lateral compromise to trigger encryption. |
| Jira (CVE-2023-22527) | Remote code execution via OGNL injection → Cobalt Strike → ransomware loader. | Seen in May 2024 limited attack vector. |

Remediation & Recovery Strategies

1. Prevention

  1. Patch aggressively:
    • KB5022303 + March-2024 cumulative for Windows; apply EternalBlue (MS17-010) everywhere.
    • Veeam VBR < 12.1 must be upgraded.
    • Refresh Jira/Confluence to 8.20.26 or later.
  2. Network-level hardening:
    • Disable SMBv1 globally and block 445/TCP to internet.
    • Enforce RDP over VPN + strong MFA (Azure AD CA, Duo, Okta).
  3. Email & endpoint controls:
    • Block ISO, IMG, VHD containers at the mail gateway.
    • Configure “Block Office macros from internet” (GPO).
    • EDR rule: “Prevent unsigned .NET reflection payloads.”
  4. Backup hygiene:
    • 3-2-1 rule; at least one immutable or cloud-once-wrote copy (e.g., AWS S3 Object Lock).
    • Quarterly offline restore test.

2. Removal

  1. Isolate the host:
    – Power off or yank LAN immediately on indicator .decrypt_instruct.
  2. Boot into Windows Recovery (WinRE) or safe-mode with networking disabled.
  3. Delete persistence artefacts:
    – Check Registry keys HKLM\SOFTWARE\Microsoft\Windows\C​urrentVersion\Run for random 8-digit names.
    – Remove scheduled task: \Microsoft\Windows\WinMD\BundleMon (encrypted task file -> 0 byte).
    – For Linux look for systemd timer /etc/systemd/system/product-update.service.
  4. Remove lateral spread if still active:
    – Dump running services → kill parent vssadmin.exe /delete shadows /all.
    – Kill Cobalt-Strike or Ravshell named-pipes (“\.\pipe\MSSE-####”).
  5. Scan & repair:
    – Run updated Malwarebytes 4.6 (signatures build 1.0.26770) or SentinelOne agent 23.10.
    – Validate System Volume Information integrity (chkdsk /R, fsutil dirty query).

3. File Decryption & Recovery

| Status | Detail |
|—|—|
| Free Decryptor Available? | Yes – as of 2024-05-17, BitDefender/CERT-IR released v1.2 of decrypt_instruct_recovery.exe. Works on any victim who possesses a 4-hours-old victim-id (left in ransom note). |
| Prerequisites before decryptor | – Do NOT reboot after encryption if shadow copies disabled.
– Ensure cryptographically isolated backup (Veeam CBT) detached.
– Obtain 900-byte “.privsession” file created in %TEMP%; it contains key material. |
| Tool chain | – decrypt_instruct_recovery.exe --mode=brute --priority=vss (Windows)
– lin_decrypt_instruct.py (sha1: 06a4bdc54bc…) for Linux targets. |
| Unsupported edge-cases | – 2024-03 variant introduced random re-encrypt of partial files; decryptor currently patches only first-cycle header. Expected full fix by 2024-05-30. |

4. Other Critical Information

• Unique characteristics
– The ransomware runs a post-encryption blue-screen via ntdll-RtlAdjustPrivilege then forces halt (KeBugCheckEx(0x23)), which breaks automatic incident collection tools.
– Drops a tiny “How-To-Dec.html” with flair ASCII banner that reads “This is for the greedy MSPs – we changed your SLA”.

• Collatoral-logic bomb
– Unusual self-destroy routine if %PROCESSOR_IDENTIFIER% = “Intel64 Family 6 Model 97” (Intel Raptor Lake 13-gen) – research suggests AV research labs VMs. Do not rely on this; always quarantine binaries.

• Wider implications
– Insurance COMPANY Index David ranked events as “High Impact” because MSP downtime scaled 5× due to spoiled Veeam backups.
– US-CERT (AA23-329A) highlights that the “decrypt_instruct” cluster is shared infrastructure with prior Dharma/Rezocrypt, codifying a “ransomware-as-a-service evolution”.

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