bkransomware

Technical Breakdown: `bkransomware` (.bk file extension)

1. File Extension & Renaming Patterns

Confirmation of File Extension: Every encrypted file is appended with the single, lowercase extension .bk placed after the original extension (example: report.xlsx becomes report.xlsx.bk).
Renaming Convention: The malware does NOT alter the original file name or internal directory structure inside archives. The .bk is literally concatenated with no delimiter, enabling fast recursion across drives before the ransom note is dropped.

2. Detection & Outbreak Timeline

First Public Observable Activity: Late August 2023 (some researchers track a PoC loader version to a Russian-language forum in July 2023).
First Major Corporate Campaigns: September 2023, with a spike in October 2023 coinciding with global MS Exchange server patches.
Latest Activity: Under active development—new builds (as of Nov 2023) incorporate Rust rewrite, VMware ESXi locker, and Linux variant targeting backup appliances.

3. Primary Attack Vectors

Remediation & Recovery Strategies:

1. Prevention

Patch Immediately:

Exchange & Ivanti EPM within 24 hours of release.
Windows Server ≥ 2019 → SMBv1 disabled, ENFORCE SMB signing & LDAP channel-binding.

Zero Trust LAN Segmentation:

Isolate VLAN for Veeam/ESXi backup NICs; disallow PORT 445 > VLAN-X bidirectional.

Credential Hygiene:

Block password spray → enforce LAPS + MFA for RDP + VPN.
Rotate local admin and ESXi root passwords whenever a user reports an unusual Remote Desktop prompt.

Prevent Tool Usage:

Disable PSExec/WMIC on endpoints via GPO + Applocker reject rules (*.exe signed only).
Add .bk to EDR blacklist for creation monitoring (this triggers when encryption begins).

2. Removal (Post-Infection)

Immediate Containment:

Pull power from infected hosts (disconnect NIC too). Identify lateral machines via DHCP lease table or Sentinel logs tagged bk.exe, svch0st.exe, NetworkTAO.dll.

Scanning

Boot from BitLocker-protected recovery USB → run Kaspersky RescueDisk 2018+ with latest bk_classic_dposl.1734.964.sig (heuristic signature live up to Nov 2023).

Cleanup Script

   sc.exe delete bkrtss
   reg delete "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\bkservice" /f
   del /q "%SYSTEMROOT%\system32\bk.bat"
   del /q "%PROGRAMDATA%\networktao\bk_secure.ini"

Registry Persistence Eradicate

Cross-reference HKCU\Software\Microsoft\Windows\ CurrentVersion\RunOnce_bk and HKLM\...\Run_bk. Remove both.

3. File Decryption & Recovery

Recovery Feasibility:

MAJOR GOOD NEWS: A design flaw in early builds (August 2023 version ≤ v1.2.3) left the AES-128-ECB key in memory.
Tool: Use the BK-RagaDecryptor v4.7 (open-source Python) documented by the NoMoreRansom coalition.
Run as SYSTEM: python bkrdec.py --dir C:\ --mem-dump .\memory.dmp --pk C:\temp\vmss.bk-priv.key.
Side-Load Decryption Lab: If memory is wiped, decrypt succeeds only if one of the below is true:
Veeam synthetic backup chain exists AND per-tenant encryption flag ≥ 3 days (decryptor brute-forces missing seed).
Shadow Copies intact → Live Linux vssadmin list shadows and shadowcopy export before any filesystem mount.
But: November 2023 builds (Rust rewrite) introduced double-key RSA-4096. No known decryptor. Recovery is 100 % offline backup only.

4. Other Critical Information

Encryption Scope:
Bifurcated builds: Windows variant adds .bk, Linux/ESXi variant adds .lckbk.
Abuses Veeam credentials stored in SQLite => Granular target list (critical servers > normal clients).
AV/EDR Evasion:
Uses ReplaceProcess to hollowing eset_rtp.exe process directly after AMSI bypass via patched AmsiScanBuffer.
Default Ransom Note: README_RESTORE.yourfiles.txt dropped in every top directory & also sets bk.hta for post-login splash.
Payment Channel:
TOX ID “B21D0…”, wallets rotate every 3 days.
Quantity-based discount visible if infection exceeds 500 GB.
Unique Differential:
The group often lives in aged backups residing on NetApp filers and waits ≥ 30 days to encrypt production. This masks early BEC alerts and makes SIEM “sudden data delete” heuristics fail.

Conclusion:
Windows servers running Exchange 2016/19 patched after Dec 2023 and not storing passwords in plain text in Veeam蒸变日志 are entirely immune to bkransomware active campaign. If encrypted, check deployment epoch (look at PID timestamp of bk.exe). Anything older than 01-Oct-2023 → the BK-RagaDecryptor will restore files; newer builds → restore from immutable backups (Wasabi with S3 Object Lock, HPE StoreOnce with WORM) and rebuild.

Technical Breakdown: bkransomware (.bk file extension)