How to remediate – OpenVPN Server Detection

1. Introduction

An OpenVPN server is listening on a remote host, indicating it’s actively running and accepting connections. This means an attacker could attempt to connect to the VPN and potentially gain access to your network if security isn’t properly configured. Systems commonly affected are servers running Linux, Windows or other operating systems with OpenVPN installed. A successful attack could compromise confidentiality, integrity, and availability of data on the protected network.

2. Technical Explanation

The remote host is running an OpenVPN server in either TLS or preshared key mode. This means it’s configured to accept incoming VPN connections using one of these authentication methods. An attacker could exploit weak configurations, such as a compromised pre-shared key or vulnerabilities in the TLS implementation. The precondition for exploitation is network connectivity to the OpenVPN server port and valid credentials or a successful key exchange.

• Root cause: The host has an OpenVPN server configured and listening for connections.
• Exploit mechanism: An attacker attempts to connect to the VPN server using valid credentials, or by exploiting vulnerabilities in the TLS handshake process if TLS is used.
• Scope: Any system running OpenVPN Server on Linux, Windows, macOS, or other supported platforms.

3. Detection and Assessment

You can confirm an OpenVPN server is running by checking for listening ports and examining the service configuration. A thorough method involves analysing the OpenVPN configuration file.

• Quick checks: Use `netstat -tulnp` (Linux) or `Get-NetTCPConnection | Where-Object LocalPort -eq 1194` (PowerShell) to check for a process listening on port 1194, the default OpenVPN port.
• Scanning: Nessus plugin ID 36857 can detect OpenVPN server instances. This is an example only and may require updates.
• Logs and evidence: Check system logs (e.g., `/var/log/syslog` on Linux, Windows Event Viewer) for OpenVPN-related events.

netstat -tulnp | grep openvpn

4. Solution / Remediation Steps

4.1 Preparation

• Ensure you have access to the OpenVPN configuration file and understand its settings. A roll back plan is to restore from the backup or restart the stopped OpenVPN service.
• A change window may be needed, depending on your organisation’s policies. Approval from a senior administrator might be required.

4.2 Implementation

1. Step 1: Review the OpenVPN configuration file (usually `/etc/openvpn/server.conf` on Linux) for security best practices.
2. Step 2: If using pre-shared keys, switch to TLS authentication with strong certificates and key management.
3. Step 3: Ensure all clients are configured to use the updated authentication method.
4. Step 4: Restart the OpenVPN service to apply changes.

4.3 Config or Code Example

Before

proto udp
dev tun
ca /etc/openvpn/easy-rsa/pki/ca.crt
cert /etc/openvpn/easy-rsa/pki/issued/server.crt
key /etc/openvpn/easy-rsa/pki/private/server.key  # This shows key on disk
dh /etc/openvpn/easy-rsa/pki/dh.pem
server 10.8.0.0 255.255.255.0
ifconfig-pool-persist ipp.txt
push "redirect-gateway def1 bypass-dhcp"
push "dhcp-option DNS 8.8.8.8"
push "dhcp-option DNS 8.8.4.4"
keepalive 10 120
tls-auth ta.key 0 # This shows tls auth key on disk
cipher BF-CBC        # Weak cipher
comp-lzo
persist-key
persist-tun
status openvpn-status.log
verb 3

After

proto udp
dev tun
ca /etc/openvpn/easy-rsa/pki/ca.crt
cert /etc/openvpn/easy-rsa/pki/issued/server.crt
key /etc/openvpn/easy-rsa/pki/private/server.key
dh /etc/openvpn/easy-rsa/pki/dh.pem
server 10.8.0.0 255.255.255.0
ifconfig-pool-persist ipp.txt
push "redirect-gateway def1 bypass-dhcp"
push "dhcp-option DNS 8.8.8.8"
push "dhcp-option DNS 8.8.4.4"
keepalive 10 120
cipher AES-256-CBC # Stronger cipher
comp-lzo
persist-key
persist-tun
status openvpn-status.log
verb 3

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent issues related to OpenVPN server exposure. Least privilege limits the impact of a compromised account. Secure defaults ensure strong encryption and authentication are used from the start. Patch cadence ensures known vulnerabilities are addressed promptly.

• Practice 1: Apply least privilege principles to OpenVPN user accounts, limiting their access to only necessary resources.
• Practice 2: Use secure defaults in your OpenVPN configuration, such as strong encryption ciphers and TLS authentication.

4.5 Automation (Optional)

# Example Ansible playbook snippet to update cipher in OpenVPN config file
- name: Update OpenVPN Cipher
  lineinfile:
    path: /etc/openvpn/server.conf
    regexp: '^cipher BF-CBC'
    line: 'cipher AES-256-CBC'
  become: true
  notify: Restart OpenVPN

5. Verification / Validation

Confirm the fix by checking the OpenVPN configuration and verifying that TLS authentication is enabled. A smoke test confirms basic VPN connectivity.

• Post-fix check: Run `grep cipher /etc/openvpn/server.conf` and confirm the output shows `cipher AES-256-CBC`.
• Re-test: Re-run `netstat -tulnp | grep openvpn` to ensure the server is still listening, but with updated security settings.
• Smoke test: Attempt to connect to the VPN using a valid client configuration and verify you can access resources on the protected network.
• Monitoring: Monitor OpenVPN logs for authentication failures or unusual activity. For example, search for failed TLS handshakes.

grep cipher /etc/openvpn/server.conf

6. Preventive Measures and Monitoring

Update security baselines to include strong OpenVPN configuration settings. Implement checks in your CI/CD pipelines to prevent insecure configurations from being deployed. A regular patch review cycle ensures vulnerabilities are addressed promptly.

• Baselines: Update your security baseline or policy to require TLS authentication and strong encryption ciphers for all OpenVPN servers.