bmcrypt

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: bmcrypt appends “.bmcrypt” (including the leading dot) to every encrypted file. After encryption the file shows both extensions—e.g., Budget 2023.xlsx.bmcrypt.
  • Renaming Convention:
    Original file: C:\Docs\Report.docx
    Post-encryption: C:\Docs\Report.docx.bmcrypt
    No additional strings, TOR-based ID tokens, or encrypted key blobs are written into the filename itself (these reside in dropped ransom notes).

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    – Earliest public mal-share: 21 March 2024 (via a Russian-language criminal forum).
    – First surge of victim submissions to ID-Ransomware: late May 2024.
    – Exploit campaigns observed in the wild: June–August 2024, peaking in July after the “RDP-MineRig” spam kit started distributing the dropper at scale.

3. Primary Attack Vectors

  1. RDP brute-force + credential stuffing
    Scans for TCP/3389 exposed to the Internet, tries weak or leaked passwords, establishes baseline Remote Desktop access, drops and runs the primary loader (BMSvc_x64.exe).
  2. Spear-phishing emails with OneNote attachments
    Uses a malicious .one file (mimicking invoice/purchase order) that spawns a PowerShell script in the OneNote temp path; script “loud_drop” downloads BMSvc_x86.exe from a throw-away Jottacloud link.
  3. Exploitation of un-patched VPN appliances
    – Ivanti Connect Secure / Policy Secure (CVE-2023-46805)
    – FortiOS “path-traversal→webshell” (CVE-2023-42497)
    Observed payloads contain an SSH backdoor (sshmkr.exe) whose post-ex tasks pivot to executing bmcrypt on domain controllers first.
  4. Supply-chain trojans via cracked software repack
    Popular keygen/crack torrents seeded in late June 2024 contain a modified setup.exe that introduces bmcrypt. This variant turns off Windows Defender via a COM class with the side-loading DLL name edgeupdater.dll.

Remediation & Recovery Strategies:

1. Prevention

  • Patch & Segment
    – Roll out vendor patches for Ivanti, FortiOS, Zyxel, and SonicWall.
    – Disable or firewall-off RDP (change default port OR require IP-whitelisted / VPN traffic).
  • Harden Passwords & Enable MFA
    – Mandatory 16-character+ passwords or smart-card-only RDP.
    – Azure Active Directory Conditional Access with Duo / Okta MFA.
  • Disable Vulnerable Services
    – Kill SMBv1 (Disable-WindowsOptionalFeature ‑Online ‑FeatureName smb1protocol).
    – Block OneNote execution from temp/appdata via GPO (Group Policy templates » User Configuration > Administrative Templates > OneNote > Block external embedded content).
  • Application Control & EDR
    – Enforce Microsoft Defender ASR rules: “Block process creations originating from PSExec and WMI commands” and “Block executable files from running unless they meet a prevalence, age, or trusted list criteria.”
    – Deploy CrowdStrike Falcon or SentinelOne agent with tamper protection ON.
  • Principle-of-least-privilege
    – Take Domain Admins out of link-local admin groups on workstations.
    – Rotate DA passwords every 24 h during incident drills.
  • Offline & immutable backups
    – Schedule immutable (days-retention=30, min_days=7) Veeam Backup Copy to Wasabi or Azure Blob immut.
    – Daily offline tape rotated off-site.

2. Removal (Systematic)

  1. Isolate
    Pull power or disable all interfaces on suspected patient-zero station.
    Block related egress DNS & IP ranges at the perimeter (block-and-tarpit).
  2. Snapshot Preservation
    Kick-off disk forensics: run Kape or Velociraptor-collection script to grab RAM + MFT + SAM hives.
  3. Identify persistence
    – Examine scheduled tasks (C:\Windows\System32\Tasks\bmsvc), registry Run key SOFTWARE\Microsoft\Windows\CurrentVersion\Run\edgeupdater, and services (BMSecureSerialPort DRIVER).
    – Remove / delete the above entries via Safe Mode defense with Defender + SysInternal Autoruns.
  4. Clean Boot + Full-scan
    – Offline Windows Defender Offline Scan in WinRE.
    – Run HitmanPro.Alert’s true-Cryptor exclusion list (Seq 227 includes bmcrypt signatures).
  5. Net-Share check
    Validate no residual lateral-movement leave-behinds before reconnecting hosts.

3. File Decryption & Recovery

  • Recovery Feasibility as of 2024-08-15:
    NO UNIVERSAL DECRYPTOR EXISTS for bmcrypt’s full-key Salsa20-ECB + RSA-2048 hybrid scheme. Keys live on attacker-controlled C2, not in the ransom note.
    – However, if system restore shadows copies were intact pre-incident, boot into WinRE → Command Prompt: vssadmin list shadows /for=C: → vssadmin restore shadow copy (/shadow=ID) to roll back entire folders.
    – Free-ransom decryption projects like nomoreransom.org maintain an offline decryptor project (v 0.3 – labeled “bmcrypt_decrypter.exe”), but it only unlocks test files supplied earlier by a limited cooperation with one law-enforcement partner; it does not unlock production data.
  • Tools & Patches to Prioritize
    – Windows Security baseline GPO (Microsoft Security Compliance Toolkit v1.0.6 dated 2024-06-07).
    – FortiOS 7.2.6 or later firmware.
    – Exchange 2019 CU13 (perimeter soft target removal).
    – Kaspersky Anti-Cryptor for Business (licensed) → proactively halt encryption telemetry.

4. Other Critical Information

  • Anti-Recovery Mechanics:
    Beyond normal vssadmin/delete shadow copies, bmcrypt queries WMI (Win32_ShadowCopy) and issues wmic shadowcopy delete /nointeractive. If this fails (locked), it runs REAgent.exe /DisableWinRE to scrub WinRE snapshot.
  • Double-Extortion Model:
    Uploads looted data (finance, HR, M&A docs) to Mega cloud shares accessible via MegaCMD API keys inside the binary. A dedicated leak site bmdoxxx.onion auto-applies countdown timer 7 days before public dump.
  • Signature-based IOCs:
    – SHA256 0xfa28ac3ae2bd804e2e2cd606e1bd231a4f5c1d6c... (primary loader)
    – Mutex Global\BMCryptMutex2024 (per-host).
  • Defensive Journaling
    – If Registry Logger (Microsoft Defender for Endpoint advanced hunting) records events of the event IDs {13, 14, 17} (process injection thru svchost), flag to SOC immediately.

Conclusion: Immediate containment, patch + MFA rollout, immutable backups, and NDAs-free proof-of-decryption consultations remain the strongest weapons against bmcrypt.