Ensure there is no unrestricted inbound access to TCP port 137 (NetBios)

Security & Compliance

TCP port 137 is used for the NetBIOS Name Service, which is a legacy protocol used to provide name resolution services on local area networks. It is commonly used on Windows-based networks for file sharing and printer sharing. However, it can also be used by attackers to perform reconnaissance and launch attacks. Ensuring that there is no unrestricted inbound access to TCP port 137 is important to prevent unauthorized access and potential attacks on the network.


To remediate the issue of unrestricted inbound access to TCP port 137 (NetBios), you can follow these steps:

  1. Identify the system where the port 137 is open and determine if it is necessary to keep it open.
  2. If the port is not needed, close it using the firewall or other access control mechanisms.
  3. If the port is required for specific purposes, configure the firewall or access control mechanisms to restrict access only to authorized IP addresses or network segments.
  4. If the system is a Windows machine, ensure that the SMB service is not exposed to the Internet or untrusted networks. To do this, disable NetBIOS over TCP/IP in the network adapter settings and enable SMB signing.
  5. If the system is a Linux machine, ensure that Samba is configured securely and restrict access only to authorized users and IP addresses.
  6. Monitor the system and its logs for any unusual activity or attempted intrusions.

Overall, the key to remediation is to minimize the risk of unauthorized access to the system through port 137, while still allowing necessary access for legitimate users and processes.

Enforced Resources
Note: Remediation steps provided by Lightlytics are meant to be suggestions and guidelines only. It is crucial to thoroughly verify and test any remediation steps before applying them to production environments. Each organization's infrastructure and security needs may differ, and blindly applying suggested remediation steps without proper testing could potentially cause unforeseen issues or vulnerabilities. Therefore, it is strongly recommended that you validate and customize any remediation steps to meet your organization's specific requirements and ensure that they align with your security policies and best practices.