How to remediate – Enumerate IPv6 Interfaces via SSH

1. Introduction

The vulnerability “Enumerate IPv6 Interfaces via SSH” allows an attacker with valid credentials to identify IPv6 interfaces configured on a remote host. This information can be used in further reconnaissance and potentially lead to network mapping, targeted attacks, or exploitation of other vulnerabilities. Systems running SSH are usually affected. A successful exploit could result in information disclosure, impacting confidentiality.

2. Technical Explanation

Nessus connects to the remote host via SSH using supplied credentials and enumerates any network interfaces configured with IPv6 addresses. This is possible because SSH allows interface enumeration upon connection. The vulnerability does not involve unauthorized access; it requires valid login details. An attacker could use this information to identify potential attack surfaces on the system’s network configuration.

• Root cause: SSH protocol allows enumeration of interfaces when a connection is established with valid credentials.
• Exploit mechanism: An attacker connects to the target host via SSH using legitimate credentials and uses standard commands or tools to list IPv6 interfaces.
• Scope: Systems running OpenSSH are affected. Specific versions are not explicitly identified in this report, but any system allowing SSH access is potentially vulnerable.

3. Detection and Assessment

You can confirm if a system is vulnerable by checking for the presence of IPv6 interfaces or reviewing SSH logs for interface enumeration activity.

• Quick checks: Use the command ip addr show to list all network interfaces, including IPv6 addresses.
• Scanning: Nessus (plugin ID not provided) can detect this vulnerability. Other scanners may have similar capabilities.
• Logs and evidence: Check SSH logs for connection attempts and interface enumeration activity. Log file location varies by system but is often in /var/log/auth.log or /var/log/secure.

ip addr show

4. Solution / Remediation Steps

Disable IPv6 if it’s not actively used, or disable unused interfaces to reduce the attack surface.

4.1 Preparation

• Ensure you have alternative access methods in case of issues. A roll back plan is to re-enable IPv6 or the disabled interfaces.
• Change windows may be required depending on business needs and impact. Approval from system owners may be needed.

4.2 Implementation

1. Step 1: Edit the SSH configuration file (usually /etc/ssh/sshd_config).
2. Step 2: Add or modify the line AddressFamily inet to restrict SSH connections to IPv4 only.
3. Step 3: Restart the SSH service using sudo systemctl restart sshd (or equivalent command for your system).

4.3 Config or Code Example

Before

# No AddressFamily setting present, allowing both IPv4 and IPv6 connections

After

AddressFamily inet

4.4 Security Practices Relevant to This Vulnerability

• Least privilege: Limit SSH access to only authorized users and systems.
• Secure defaults: Configure SSH with strong security settings, including disabling unused features.

4.5 Automation (Optional)

No automation is provided as this requires careful configuration changes specific to each system.

5. Verification / Validation

Confirm the fix by checking that SSH connections are restricted to IPv4 and re-running the detection methods.

• Post-fix check: Use ip addr show again; only IPv4 addresses should be listed for the SSH interface.
• Re-test: Run Nessus or a similar scanner to confirm that the vulnerability is no longer detected.
• Smoke test: Verify that authorized users can still connect via SSH using IPv4.
• Monitoring: Monitor SSH logs for any unexpected connection attempts or errors.

ip addr show

6. Preventive Measures and Monitoring

Regularly review system configurations, apply security baselines, and monitor SSH logs to prevent similar issues.

• Baselines: Update your security baseline or policy to include secure SSH configuration settings.
• Pipelines: Implement automated checks in CI/CD pipelines to ensure that SSH configurations meet security standards.
• Asset and patch process: Maintain a regular patch cycle for all systems, including SSH software.

7. Risks, Side Effects, and Roll Back

• Risk or side effect 1: Disabling IPv6 could break applications or services that require it.
• Roll back: Remove the AddressFamily inet line from the SSH configuration file and restart the SSH service to re-enable IPv6.

8. References and Resources

No specific references are available in this context.

• Vendor advisory or bulletin: Not provided.
• NVD or CVE entry: Not provided.
• Product or platform documentation relevant to the fix: Refer to your SSH software’s documentation for configuration options.