blm

[Content by Gemini 2.5]

Ransomware Family Guide: “BLM”
(Responsible for the .blm extension suffix on encrypted files)

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: All encrypted files receive the suffix .blm (e.g., report.xlsx.blm, photo.jpg.blm).
  • Renaming Convention: BLM rarely changes the core filename. Instead, it simply appends .blm after the original extension. Windows users therefore see double extensions that can be misleading (“report.xlsx” appears to be the only name in Explorer if “Hide file-name extensions” is enabled).

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: Samples bearing the .blm suffix surfaced 12 June 2020 and peaked during July–August 2020. Active campaigns ceased by late Q3-2020, but re-appear sporadically in phishing bursts tied to the TrickBot botnet.

3. Primary Attack Vectors

| Vector | Details & Historical Examples |
|—|—|
| Spear-phishing | Malicious spam with ISO or ZIP attachments disguised as invoices/export documents that drop the BLM loader. |
| RDP & VPN Brute-force | Attackers leverage stolen credentials or old RDP vulnerabilities (BlueKeep CVE-2019-0708, FortiOS CVE-2018-13379) to gain footholds and deploy BLM manually. |
| Lateral Movement via SMB | Post-exploitation tools (Cobalt Strike, Mimikatz) use SMB for lateral spread; no documented use of EternalBlue in-the-wild for BLM, but the capability can be added by affiliates. |
| Drive-by downloads | Water-hole sites seeded with RIG, Fallout, or CloudEyek exploit kits; the exploit kits in turn drop the BLM downloader (notably Flash/Java exploits CVE-2018-15982 & CVE-2019-1096). |

Remediation & Recovery Strategies:

1. Prevention

  • Adopt MFA for all remote-connection services (RDP, VPN, web mail).
  • Delete SMBv1 and block TCP/139 & TCP/445 egress where possible.
  • Disable Office macros by default; enforce “Block at First Sight” via Windows Defender AV.
  • Segment internal networks; isolate critical workstations from general user VLANs.
  • Deploy Microsoft Defender for Endpoint, SentinelOne, CrowdStrike, or equivalent EDR tuned to block remote-service abuse and to detect Cobalt-Beacon traffic.
  • Keep Windows, Fortinet, Pulse, and Java runtimes fully patched (patch window < 7 days for Internet-facing devices).

2. Removal

  1. Isolate the victim host(s) from the network immediately (unplug cable or disable VLAN).
  2. Boot to Safe Mode with Networking to prevent further encryption; some newer BLM samples are tied to HKCU\Run keys—Safe Mode prevents these.
  3. Acquire a forensic image (optional but recommended) before any cleanup.
  4. Scan & Clean – Run up-to-date Windows Defender Offline or Malwarebytes AM 4.x; remove any C:\Users%USER%\AppData\Roaming\svchost.exe or similarly named binaries hiding in %TEMP%.
  5. Clean Registry Hives (look under RunOnce for traces such as soft shutdown.exe) and remove scheduled tasks or services created by the BLM payload.
  6. Apply reboot and confirm the malware binary no longer re-spawns with Process Explorer.

3. File Decryption & Recovery

  • Recovery Feasibility: Despite being ChaCha-20 + RSA-4096 encryption, free decryption became possible.
  • Available Tool:
    – Kaspersky BLM Decryptor (AKA decryptor_blm.exe) released August 2020 after the command-and-control server private keys were recovered by Dutch law-enforcement. It works for all observed BLM builds if you possess the matching .readme_to_decrypt.txt ransom note (which contains your victim-ID hash).
  • Operation Process:
  1. Copy infected files (or full VM) to a new folder for safety.
  2. Run decryptor_blm.exe -v -dir X:\EncryptedData (turn on verbose logging).
  3. Feed ransom note into the tool when prompted; leave system online only long enough to fetch the master key and perform decryption.
  4. Validate output: random sample files should open without the .blm suffix once complete.

4. Other Critical Information

  • Unique Characteristics:
    – Inserts [Encrypted by BLM] into each file’s alternate data stream as a fingerprint (visible with streams.exe SysInternals).
    – Self-destruct payload rolls log files into %TEMP%\blaze.log then launches wevtutil cl Security to clear event logs.
  • Broader Impact:
    – BLM’s operators were successors to the same developer group behind Zeppelin & Buran ransomware but shifted from Russian-speaking underground marketplaces to a global multi-exploit package.
    – Public health and food-supply companies were disproportionately hit in July 2020, illustrating the shift away from purely finance-as-targets.

Bottom line: BLM is no longer under active development, free decryption exists, and victims are encouraged to back up the .readme_to_decrypt.txt file tightly. Still, early network hardening and disablement of antiquated protocols remain the best defense against whatever successor strain emerges next.