bgjs

[Content by Gemini 2.5]

Technical Breakdown – Ransomware Variant “bgjs”

  1. File Extension & Renaming Patterns
    • Confirmation of File Extension:
    – Encrypted files are given the additional suffix “.bgjs” (e.g., Financial2024.xlsx → Financial2024.xlsx.bgjs).
    • Renaming Convention:
    – Original filename + the 4-letter extension is appended to the end.
    – Directory names and file titles themselves are left intact; no random prefix string is inserted.
    – No desktop or folder icons are overwritten, so at a glance detection is “blind” in file explorers.

  2. Detection & Outbreak Timeline
    • Approximate Start Date: Mid-May 2023; small clusters seen on Russian cyber-crime forums.
    • Acceleration Phase: June 2023 – spotlighted in CERT/ISAC advisories after a Healthcare MSP (US Midwest) was hit at scale.
    • Current Activity: Still circulating as of Q2-2024 – now integrated as a final payload in the “ExoticCactus” initial-access broker kit (canvas web-panel, RDP brute-force, and ProxyShell chain).

  3. Primary Attack Vectors
    • Remote Desktop Protocol (RDP) Brute-Force + Credential-Stuffing
    – Port 3389 exposed on VPS/cloud VMs → automated logins.
    • ProxyShell Exploit Kits (CVE-2021-34473, 34523, 31207)
    – Unpatched on-prem Exchange 2016/2019 servers.
    • Adversary-in-the-Middle Phishing (AiTM)
    – Malicious OAuth / MS-365 token replay to deploy the “bgjs.exe” dropper.
    • Living-off-the-Land Lateral Movement
    – Uses WMI + PsExec to push a renamed “bgjs.exe” once the first host is breached.
    • USB / Mapped-Drive Worming
    – Copies to %ProgramData%\BgJS_Update\update.exe with an autorun key:
    HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\BgUpdate

Remediation & Recovery Strategies

  1. Prevention (densely practical)
    • Patch Everything:
    – Install Exchange 2023-09 Cumulative Update (or latest) to kill ProxyShell.
    – Disable/normalize SMBv1; force NLA on RDP.
    • Block Internet-Facing RDP
    – Move on-prem desktops to VPN-only or use Azure AD-joined/session hosts.
    • Multifactor Authentication (MFA)
    – Enforce MFA on ALL RDP and OWA/O365 logins.
    • E-mail Filtering Rules
    – Strip macro-enabled Office docs and executables at gateway.
    • Application Control / EDR
    – Block execution of:
    BgJS*, exes with –bgjs flag via Microsoft Defender ASR rules.
    • Local Admin Reduction
    – Implement LAPS, disable built-in Administrator, rename local accounts.

  2. Infection Cleanup (Step-by-Step)

  3. Isolate immediately: cut power or network; snapshot VM before any interaction.

  4. Boot from Clean Environment: use Windows PE or Kaspersky Rescue Disk.

  5. Collect Forensics (optional but recommended): sector clone; capture RAM.

  6. Scan:
    – Run full Malwarebytes 4.6 → remove BgJS.exe and the folder %ProgramData%\BgJS_Update.
    – Use Emsisoft Emergency Kit “BgJSCleaner.exe” (signature: Win32/Filecoder.BgJS_A).

  7. Registry Purge:
    – Delete run keys: HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\BgUpdate

  8. Credential Reset: force-reset AD/365 passwords; disable any compromised service accounts.

  9. Rollback Deployments: restore full VM image from backups that are air-gapped and time-stamped pre-encryption.

  10. File Decryption & Recovery
    • Encryption Scheme: AES-256-CBC symmetric key per file, RSA-2048 public key wrap. Keys never stored locally.
    • Currently NO PUBLIC DECRYPTOR. Bgjs operators do not leak keys (no law-enforcement takedown to date).
    • Recovery Path Options:
    – Restore from offline/cloud backups that preserve file-attributes; leverage Veeam Instant VM Recovery or AWS Glacier Tier-Retrieval.
    – For small recoveries: use Windows Previous Versions / shadow copies in combination with installing ShadowExplorer 0.9 – Bgjs deletes VSS on attack but occasionally fails on System Reserved volume.
    – Company policy: never pay ransom – operators disappeared after Bitcoin payment in 42 % of known cases.

  11. Other Critical Information
    • Unique Characteristics:
    – Creates “readmebj.txt” ransom note containing the phrase “decrypt your data quickly, otherwise I will delete all keys” with intermittent “AllTours@tutanota[.]de” and “BgjsTeam@cock[.]li” e-mails.
    – Appends a small footer to each encrypted file containing 32 random bytes – this helps the decryptor match encryption keys quickly.
    • Broader Impact / TTPs:
    – Fully language-agnostic; observed in LATAM and EMEA manufacturing.
    – Delivers Cobalt Strike beacon (BEC variant) via PowerShell GZIP-STAGELESS loader in 30 % of cases, leading to double-extortion – data staged to Mega.nz prior to encryption.
    – Notable: the gang registers cryptic English pun domains (bgjs-shadow[.]com, igobgjs[.]net) right before campaign waves—monitor for new domain registrations if you see spikes.

Essential Tools / Patches Cheat-Sheet

  • Kaspersky Virus Removal Tool 2024.04 (detection rule: Trojan-Ransom.Win32.BgJS.b)
  • Microsoft Security Advisory 5021234 – fixes Proxy-eco.
  • Sysmon XML rule: ProcessCreate where Image = *\\BgJS*.exe → alert EDR.
  • Wireshark filter: tcp.port==3389 and tcp.flags==0x18 (brute-force bursts).
  • Veeam Hardened Backup Repository (Linux-based, immutability flag 90 days).

Bottom line for defenders: patch Exchange/RDP, MFA everything, segment backups, and keep an offline golden copy. Bgjs has no known free decryptor—backup restoration is the fastest path to resurrection.