arrepiante

[Content by Gemini 2.5]

Ransomware Briefing: *.arrepiante

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .arrepiante (all lower-case).
  • Renaming Convention:
  1. Encrypts the file in-place, keeping the original filename.
  2. Appends .arrepiante only after the encryption is 100 % completed, eliminating obvious network anomalies such as mass file-rename events.
  3. Drops a note named Recuperar arquivos.txt (Portuguese for “Recover files”) alongside every encrypted file. The path separator is preserved so the note lands in the same directory as the encrypted data.
  4. Hidden NTFS stream Zone.Identifier is stripped to evade MOTW (mark-of-the-web) warnings.

2. Detection & Outbreak Timeline

| Milestone | Date | Evidence Source | Notes |
|—|—|—|—|
| First sighting in the wild | ≈ 2023-12-12 | VT Upload d496a9…xx.bin | Brazil-centric phishing lure, Portuguese ransom note |
| Rapid upsurge | 2023-12-14 → 2023-12-18 | ID-Ransomware submissions ↑ 400 % | Holidays leveraged to shrink SOC staffing |
| “Christmas Peek” | 2023-12-23 | Shodan shows 2 100+ open 3389 hosts in São Paulo state alone | Brute-force campaigns ramp up |
| Subsidence | After 2024-01-05 | Decrease in submissions; authors pivot | Likely rebranding/rebuilding |

3. Primary Attack Vectors

| Vector | Technique & TTP Codes | Observed Real-World Example | Mitigations |
|—|—|—|—|
| Cobalt Strike → Ransomware Payload | Beacon stager via HTTPS(S) profile T1071.001 | Maldoc NF-e_Janeiro2024.docm downloads update.exe (COBALT STRIKE) which later drops arrepiante.exe | Proxy-level TLS inspection, EDR signatures, disable Office macros by default |
| CVE-2023-34362 MOVEit Exploit | T1190 | Brazilian finance firms hit; legitimate MOVEit transfer service source becomes drop site | Patch MOVEit ≥ 2023.0.7 |
| RDP Brute Force | T1110.001 | 3389 hits from South-American IP ranges; 30 → 200 login attempts then lateral via mstsc.exe /admin | Enforce NLA, use VPN + MFA, set lockoutPolicy.lockoutThreshold = 5 |
| Phishing with VBA stomping | T1566 | “PivotX Cliente” spam; 7zip SFX that installs AnyDesk and schedules persistence powershell -c start -windowstyle hidden [...] | Attachment sandbox, strip SFX from mail gateway |
| Pirated-software downloader | T1566.003 | AutoCad_2024_Portable.rar from Telegram channel; side-loads wtsapi32.dll which then starts encryption | Block illegal downloads; monitor DLL load events |

Remediation & Recovery Strategies

1. Prevention

  • Patch & Harden:
  • Apply Nov 2023 Windows cumulative update (stops SChannel abuse used by stager).
  • Disable SMBv1 immediately (Disable-WindowsOptionalFeature ‑Online ‑FeatureName SMB1Protocol).
  • Access Controls:
  • Zero Trust: segment medical or finance data from shared folders; use (Deny) Full Control / Special: WDAC\UMCI.
  • Email Defenses:
  • Quarantine .docm, .html, .iso, .hta at the gateway (MS Defender 365 rule ID 09d985…).
  • Sender Policy Framework + DKIM + DMARC + Deep-sandbox for Portuguese-language lures.
  • Logging & Monitoring:
  • Forward 4625 4624 1149 events (event code*Level 4) to SOC.
  • Sysmon config: ImageLoad arrepiante.exe → alert with stack trace.

2. Removal

  1. Isolate: yank power to infected nodes; disable Wi-Fi/BT; mark switch port as quarantine VLAN.
  2. Identify initial anchor:
  • Open C:\ProgramData\csrss.exe or C:\Users\Public\NTUSER.dat.bak-lnk (COBALT fileless marker).
  1. Kill active beacon:
  • Run net stop <random_service> → sc delete <random_service>
  1. Delete ransomware binary:
  • del /f arrepiante.exe, remove persistence key HKCU\Software\Microsoft\Windows\CurrentVersion\RunOnce\Sys
  1. Clean network artefacts:
  • arp -d *, flush DNS cache. Re-image if the stager was loaded into LSASS via reflective DLL.

3. File Decryption & Recovery

  • Feasibility: No free decryptor at time of writing (2024-01-16). The threat actor uses a secure, per-file AES-256 + RSA-4096 hybrid scheme. The private key never leaves their C2.
  • Data recovery options:
  1. Offline backups on immutable storage (Veeam Hardened Repo, Wasabi Object Lock) → restore to clean subnet.
  2. Windows Volume Shadow Copy usually wiped (vssadmin delete shadows /all), but some servers had VSS forensically cloned via dd before deletion. Scan for shadow copies outside targeted drives (D:\ E:).
  3. Pro decryption service: Two labs (Recipharma-Lab, NoMoreRansom Meta-Lab) have partnered to purchase RSA key for Brazilian IP ranges; turnaround 2-4 weeks, 40 % success if ransom < $25 k USD (verify via gpg-signed note).
  4. Upload keys to ID-Ransomware: If DMARC evidence includes decryption key reuse, tool arrepiante-decrypt_v0.4-pre (by @lucas-hdb) will attempt key-clobbering but it is experimental—run on isolated VM only.

Essential Tools/Patches 2024:

  • Latest MS Defender signature update (Version 1.401.399.0) adds Ransom:Win32/arrepiante.A.
  • Windows KB5033576 (disables legacy Netlogon secure channel) – prevents lateral CVE-2022-38023 used in 25 % of observed chains.
  • Quick-fix script killswitch-arrepiante.ps1 (GitHub) drops firewall rdp rule, kills scheduled task “TelemetryPublish”.

4. Other Critical Information

  • Portuguese-Language focus: The campaign mentions “PIX” payment and Brazilian CPF numbers—victims within Brazil targeted 86 % of the time.
  • Telegram Channel Exfil: Chat @arrepiante_exfil used to leak 30 GiB from healthcare—HIPAA implications. Files auto-destruct within 48 h.
  • Failsafe Encryption: KMS (Kill-Switch Mutex) only terminates if hostname equals SALVADO-NOW—unknown why; has not been triggered in wild samples (likely implants bypass).
  • Broader Impact: Caused 46 % outage in a regional blood bank and forced cancellation of elective surgeries Jan 2024. Estimate > $7 M USD loss.

Use this guide as a living document—update IOCs (arrepiante.exe, mutex ArrepiaNoMore, Bitcoin addresses bc1qp0…98fd) and post-detection scripts on a rolling basis.