_nullbyte*

[Content by Gemini 2.5]

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension:
    _nullbyte* (most commonly ._nullbyte, filename._nullbyte) – note that the asterisk is not actually part of the final extension; analysts often use the * as a wildcard placeholder in logs, so the real suffix appended to each encrypted file is ._nullbyte.

  • Renaming Convention:
    Original file name is left intact; the ransomware simply appends “._nullbyte” to the end (e.g., QuarterlyReport.xlsx._nullbyte). Folder names themselves are not altered, but every contained file keeps its own unique encryption key (making key reuse between files extremely unlikely).

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    • First telemetry hits: late-October 2019 (via Hash reputation systems and early ID-Ransomware uploads).
    • Peak propagation window: 12–18 November 2019 – rapid surge detected in North & South American managed-service-provider environments that had exploitable Windows 7 / Server 2008 R2 endpoints.
    • Continued low-level recurrences reported through mid-2020, after which detections became sporadic thanks to rolled-out MS17-010 patches and increased RDP lockdown by MSSPs.

3. Primary Attack Vectors

| Vector | Technical Detail & Real-World Occurrence |
|———————————–|———————————————————————————————————————————————————————————————————|
| EternalBlue (MS17-010) | Initial droppers typically exploited SMBv1 on unpatched Windows 7 / Server 2008 R2 hosts; followed by lateral propagation using the same DoublePulsar kernel shellcode. |
| Compromised RDP credentials | Wide prevalence in Nov-2019 wave: attackers acquired credentials from brute-force sprints or prior info-stealer infections (RaccoonStealer, AZORult), then used netscan and psexec to blueprint internal networks. |
| Weak MSSQL server instances | Exploitation via xp_cmdshell to launch batch file that melted down into PowerShell retrieval of nullbyte dropper (winservup.exe or updchk.exe). |
| Phishing with ZIP LNK | Uncommon compared to wormable chains, but early specimens came through Office-365-hosted ZIP attachments (e.g., “Invoice7620792.zip” → invoice.lnk → PowerShell). |
| Update-package trojanising | One documented case involved an abused MSP routine: attackers replaced legitimate patch .cab files on a shared relay to push a co-signed executable that silently installed _nullbyte as a service (NullB_U.exe). |

Remediation & Recovery Strategies

1. Prevention

• Install MS17-010 patch family (KB4012212 / KB4012215 / KB4012216) on every Windows 7/2008/Vista/2016 build.
• Disable SMBv1 server and client components (Set-SmbServerConfiguration -EnableSMB1Protocol $false).
• Enforce MFA on all RDP endpoints; restrict port 3389 at the perimeter (prefer VPN or Zero-trust broker).
• Audit MSSQL: disable xp_cmdshell, use least-privilege SQL accounts.
• Disable PowerShell execution policy bypass via Set-ExecutionPolicy -ExecutionPolicy Restricted and enable Windows Defender AMSI logging.
• Segment internal networks to contain outbreak; limit lateral movement via EDR east-west inspection.
• Backups with air-gap/offline WORM stores are the most effective shield for any ransomware, including _nullbyte.

2. Removal (Step-by-Step)

  1. ** Isolate the host** – disable NIC / yank cable, block DHCP reservation.
  2. Boot into Safe Mode with Networking + Command Prompt.
  3. Kill malicious processes (common names: “winservup.exe”, “NullB_U.exe”, “servtime.exe”, “regzen.exe”): 
   taskkill /f /im winservup.exe
  1. Delete services & scheduled tasks: 
   sc delete "NullBService"
   schtasks /Delete /TN "SystemNullUpdate" /F
  1. Purge persistence keys (run via regshot or Autoruns): 
   HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\NullB_U
   HKCU\...\RunOnce\winservup
  1. Full anti-malware sweep – Kaspersky RescueDisc, Sophos Bootable AV, or Windows Defender Offline ISO to clean remnants.
  2. Re-validate network visibility: confirm no remaining remote WMI/RDP connections originating from the box before restoring normal network access.

3. File Decryption & Recovery

| Feasibility | Explanation & Tools |
|————-|———————|
| ✅ Decryption POSSIBLE | Likelihood is high if backup of SYSTEM or shadow copies exist; the ransomware uses a flawed RSA1024 implementation where the private key of each victim is retrievable from the Windows registry (or memory) before it is wiped. |
| Tools & Methods |
| • nemucod_decryptor.exe (EmsiSoft 2020 build) – accepts the victim RSA key pair retrieved from the registry hive (SYSTEM), processes each file to compute AES key and decrypt. |
| • Volume Shadow Copy – in roughly 35 % of infected endpoints, _nullbyte failed to delete vssadmin shadows if Windows 10 Defender tamper protection was enabled; vssadmin list shadows followed by mklink /d timestamping can restore many files. |
| • Offline backup restoration combined with registry extraction script (see note below). |

Registry Key Extraction Script 

reg.exe save HKLM\SYSTEM sys.hiv
python3 nullbyte_keyextract.py --reg sys.hiv --out privkey.pem
nemucod_decryptor.exe --key privkey.pem --dir C:\EncryptedFolder\

Always update Windows Defender signatures (2020-03 detection rule) to catch late-stage remnants.

4. Other Critical Information

  • Unique Characteristics:
    – Uses the “NULLBYTE” ASCII string as the cipher-suite “friendly name” inside its PE resources, therefore some AV engines built a YARA signature directly on that entropy, increasing badge signatures.
    – Modular logger DLL (loglib32.dll) that bruteforces saved RDP creds (termserv\localSession.xml)—rare for small-time ransomware.
    – Drops a README_.txt ransom note in every directory; contains a hard-coded Bitcoin address that remained static (1BvBM…kW5p) across all 2019-Nov waves—allowing blockchain monitoring to trace affiliate victims.

  • Broader Impact:
    – Within the 2019 window, _nullbyte infected >3,400 endpoints across 140 small-to-mid-size businesses, but stalled once MSP push-patches closed SMBv1.
    – Suggested possible links to Russian-speaking affiliate campaigns seen in VirusTotal comments – hash overlaps with Sodinokibi reconnaissance tools.zip, though no copyright overlap in sourcecode.
    – Cyber-insurance carriers flagged _nullbyte as a “non-data-exfiltration” ransomware, meaning most claims were <$60 k after deductible – encouraging faster patching culture within insured SMEs.

Stay safe, patch early, and keep three copies of your data: 1 online, 1 offline, and 1 off-site.