How to remediate – VLAN Membership Policy Server Detection

1. Introduction

The remote host is running a server for assigning switch ports to VLANs dynamically, known as a VLAN Membership Policy Server (VMPS). This server manages which devices connect to specific virtual networks based on their MAC address. A compromise could allow an attacker to redirect network traffic or gain access to sensitive data on other VLANs. This impacts the confidentiality, integrity and availability of connected systems.

2. Technical Explanation

A VMPS assigns switch ports to VLANs dynamically. The root cause is that the server may not adequately validate incoming requests for VLAN assignments. An attacker could potentially spoof a MAC address or manipulate the assignment process to gain access to an unintended VLAN. There are no known CVEs associated with this specific detection, but similar vulnerabilities exist in network management protocols.

• Root cause: missing or insufficient validation of VLAN membership requests.
• Exploit mechanism: An attacker could send a forged request to the VMPS server, claiming a MAC address that is authorised for a different VLAN than intended. This would result in the switch port being assigned to the incorrect network segment.
• Scope: Cisco switches running a VLAN Management Policy Server are affected. Specific versions depend on the software installed and configuration.

3. Detection and Assessment

Confirming whether your system is vulnerable involves identifying if a VMPS server is running, and checking its configuration for potential weaknesses.

• Quick checks: Use the command `show running-config | include vlan` to see if VLAN configurations are present.
• Scanning: Nessus plugin ID 16382 may identify systems running a VMPS server. This is an example only and should be verified.
• Logs and evidence: Check switch logs for events related to MAC address assignment or VLAN changes. Look for unusual patterns or unexpected assignments.

show running-config | include vlan

4. Solution / Remediation Steps

Fixing this issue requires careful configuration of the VMPS server and network access controls.

4.1 Preparation

• Dependencies: Ensure you have administrative access to the switch. A roll back plan involves restoring the previous configuration from the backup.
• Change window: Schedule this change during off-peak hours and obtain approval from network administrators.

4.2 Implementation

1. Step 1: Review the VMPS configuration to ensure that only trusted MAC addresses are allowed to join specific VLANs.
2. Step 2: Implement strict access controls on the VMPS server, limiting who can modify VLAN assignments.
3. Step 3: Enable logging and monitoring of all VLAN changes.

4.3 Config or Code Example

Before

vlan 10 name Users
interface GigabitEthernet0/1 trunk allowed vlan 10,20,30

After

vlan 10 name Users
interface GigabitEthernet0/1 trunk allowed vlan 10
interface GigabitEthernet0/1 switchport mode access
interface GigabitEthernet0/1 switchport access vlan 10

4.4 Security Practices Relevant to This Vulnerability

List only practices that directly address this vulnerability type. Use neutral wording and examples instead of fixed advice.

• Practice 1: Least privilege – restrict access to the VMPS server to authorised personnel only, reducing the impact if compromised.

4.5 Automation (Optional)

If suitable, provide a small script or infrastructure code that applies the fix at scale. Only include if safe and directly relevant.

# Example Ansible snippet to restrict VLAN access
- name: Restrict VLAN access on switch
  cisco.ios.ios_config:
    lines:
      - interface GigabitEthernet0/1 trunk allowed vlan 10
    parents: "interface GigabitEthernet0/1"

5. Verification / Validation

Confirming the fix involves verifying that only authorised MAC addresses can join specific VLANs.

• Post-fix check: Run `show vlan brief` and confirm that the allowed VLANs are as expected.
• Re-test: Attempt to assign an unauthorised MAC address to a restricted VLAN. The assignment should fail.
• Smoke test: Verify that users can still access network resources on their assigned VLANs.
• Monitoring: Monitor switch logs for any unexpected VLAN changes or failed assignment attempts.

show vlan brief

6. Preventive Measures and Monitoring

Suggest only measures that are relevant to the vulnerability type.

• Baselines: Update your network security baseline to include strict VLAN access controls, based on a standard like CIS benchmarks.
• Pipelines: Implement configuration management tools to enforce consistent VLAN policies across all switches.
• Asset and patch process: Review switch configurations regularly for any deviations from the approved baseline.

7. Risks, Side Effects, and Roll Back

List known risks or service impacts from the change.

• Risk or side effect 2: Restricting access may require updating documentation and training for network administrators.

8. References and Resources

Link only to sources that match this exact vulnerability.

• Vendor advisory or bulletin: https://www.cisco.com/en/US/tech/tk389/tk814/tk839/tsd_technology_support_sub-protocol_home.html
• NVD or CVE entry: Not applicable for this specific detection.
• Product or platform documentation relevant to the fix: Cisco switch configuration guides