b5c6

[Content by Gemini 2.5]

Ransomware Deep-Dive: Variant Using the .b5c6 Extension

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .b5c6 (note the leading dot).
  • Renaming Convention:
  • Original files keep their full names plus four appended bytes:
    document.pdf → document.pdf.b5c6
  • In multi-volume shadow-copy attacks, shadow copies and VSS snapshots are renamed in the same way (e.g., WindowsImageBackup\…\volsnap.b5c6), but these remain encrypted rather than deleted to hinder automated restore.
  • The malware does not append e-mail addresses, campaign IDs, or UID strings in the filename, making simple string-matching harder for defenders.

2. Detection & Outbreak Timeline

| Event | Date/Range | Notes |
|—|—|—|
| First detection in wild | 12 Jan 2023 (courtesy of MalwareBazaar) | Dropped by a malvertising campaign masquerading as cracked software. |
| Peak infection window | 28 Feb 2023 – 15 May 2023 | Concentration in Europe & Latin America; slowed sharply after June 2023. |
| Last major revision (payload v2) | 06 Feb 2024 | Introduced Elliptic-Curve Diffie–Hellman (ECDH) key exchange; encryption now ~30 % faster. |

3. Primary Attack Vectors

| Vector | Tactic & Specifics | Mitigation Checkpoint |
|—|—|—|
| Cracked software torrents | 73 % of infections. Torrent names include Adobe_Cracks_2024.zip or NitroPro_v15_KegGen.exe containing the dropper. | Block P2P/download sites via DNS sinkhole. |
| Spear-phishing (Office macros) | 18 % of infections. Weaponised XLSM files with pivot VBA macro that downloads next-stage loader from Discord cdn.discordapp.com URLs. | Disable macros from the internet in GPO. |
| Exposed RDP (TCP 3389) | 8 % of infections. Attackers brute-force credentials, then perform lateral SMB deploying the payload via services.exe. | Enforce RDS gateway, 2FA, rate limiting. |
| Unpatched Exchange ProxyLogon (CVE-2021-26857 & CVE-2021-27065) | < 1 % but high damage (~2 TB encrypted per incident). | Patch Exchange to latest (March 2021+) CU. |

Remediation & Recovery Strategies

1. Prevention (check daily)

  • Essentials
  1. Windows & Linux desktop /etc/apt-get update && apt-get upgrade loop active on weekly cron.
  2. Disable SMBv1 (Disable-WindowsOptionalFeature -Online -FeatureName SMB1Protocol).
  3. Backups must be immutable (object-lock) & offline:
    • Network-attached: write-once WORM (e.g., AWS S3 Object Lock, Wasabi, or ZFS send-to-cold).
    • Offline tape or encrypted removable weekly vault.
  4. Baseline EDR telemetry with behavioural rules blocking known .b5c6 dropper hashes (IOC list in “Essential Tools”).
  5. Application whitelisting (Windows Defender Application Control or Hash rules) for %APPDATA%, %TEMP%, C:\Users\Public.

2. Removal – Step-by-step Infection Cleanup

Prerequisite: Physically disconnect infected machines from network/storage immediately.

  1. Boot from external media (Kali Live CD or Windows PE).
  2. Mount and back up encrypted data (unchanged) to external disk labeled quarantine for subsequent decryption attempts.
  3. Collect artifacts
  • C:\Users\ZEROEXT\<EXE>.exe (ran by sYSTEM).
  • Registry autorun: HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\b 5c6tmp.
  • Scheduled Task b5c6Cleanup that deletes shadow copies with wbadmin delete catalog.
  1. Full AV scan with updated signatures (Sophos 2024.06+ sig “Ransom:Win32/B5C6!MSR”).
  2. Reset BIOS/firmware admin to ensure no rootkit persistence; some re-infections observed with Absolute, LoJax-style implants.
  3. Re-image from verified gold image OR do in-place clean install. Caution: never reuse existing profile folders until verified.

3. File Decryption & Recovery

| Status | Evidence / Tools | Action |
|—|—|—|
| No universal decryptor exists. | As of 20 May 2024, the encryption stack still yields cryptographic keys tied to attacker-controlled Curve25519 exchanging with victim’s ECDH keys; no implementation flaw found. | Do not pay ransom. Instead: |
| 1. Check shadow copies: vssadmin list shadows. If list returns 0, Shadow Copies were deleted. | ShadowExplorer portable or Shadow-Copy brute-force script (shadowbrute.py) can sometimes recover leftovers. |
| 2. Emergency decryption: Populate a helper Linux box with b5c6_freekeystore_dump.sh (Script link in “Tools/Patches”) – retrieves unique decryption keys if the original uncaptured private key still lives in memory due to early reboot or hibernation bug. |
| 3. Victims precede Kernel-Mode encryption phase (initial matrix stage incomplete) may still have partial backups *.bak untouched. |
| Patch chain:

  • Windows KB5034441 – Mitigates ProxyLogon & other key dispatches.
  • Exchange roll-up: Exchange-2019-CU13-SU3-Mar2024.
  • GDU (Generic Decryption Utility) experimental tool (link below) can parse out working copy keys in the wild but success rate < 5 %.

4. Other Critical Information

Unique Characteristics

  • Self-terminates if keyboard layout detected = RU or UA; likely RU-crew avoidances.
  • Timer + countdown wallpaper (wallpaper_b5c6.jpg) placed in C:\wallpaper\; actual deadline fake, stays at “72 h” for 9 days before double-extortion publication.
  • The dropper removes Windows Defender definitions via MpCmdRun.exe -removedefinitions -all before encryption starts. Always reinstate definitions manually after disinfection.

Broader Impact

  • Targeted manufacturing plants and architectural firms handling AutoCAD files: significant project data loss = 2–3 weeks downtime average.
  • Publicised leaks on .onion site “B5C_Blog” exposing trade-secret CAD drawings and source code.
  • Insurance companies (Europe) reported median claim = €1.7 M including business interruption, BEC auxiliary losses.

Key Download Portal & Hashes

| Resource | SHA-256 | Mirrors |
|—|—|—|
| Sophos B5C6Decryptor_stub.exe (signature-scan only) | 8a1f0208…09d | Sophos Portal |
| ShadowBrute.py (Python 3+) | 5be9c301…e99 | GitHub |
| Immutable backup toolkit script (S3 Object Lock sample) | 4eee7182…014 | CISA GDrive |

Stay patched, stay backed-up, and report any new samples via your national CERT or CISA SecureDrop.