How to remediate – Internet Key Exchange (IKE) Aggressive Mode with Pre-Shared Key

1. Introduction

The Internet Key Exchange (IKE) Aggressive Mode with Pre-Shared key vulnerability affects systems using IKEv1 for VPN connections. This configuration allows attackers to capture and crack the pre-shared key, potentially granting unauthorized access to private networks. Systems commonly affected include VPN gateways and servers running IKEv1. A successful exploit could compromise confidentiality, integrity, and availability of network resources.

2. Technical Explanation

The vulnerability occurs when a remote IKEv1 service supports Aggressive Mode with Pre-Shared key (PSK) authentication. This mode is less secure than Main Mode because it doesn’t provide the same level of protection against offline dictionary attacks on the PSK. An attacker can capture network traffic during the initial handshake and attempt to crack the pre-shared key using tools like psktaker. CVE-2002-1623 describes this issue.

• Root cause: Use of Aggressive Mode with Pre-Shared Key authentication in IKEv1.
• Exploit mechanism: An attacker captures the Diffie-Hellman exchange during the IKE negotiation and attempts to crack the PSK offline using brute force or dictionary attacks.
• Scope: Affected platforms include those running vulnerable versions of IKEv1, commonly found in VPN gateways and servers.

3. Detection and Assessment

To confirm vulnerability, check your IKE configuration for Aggressive Mode support. A thorough assessment involves capturing network traffic during a VPN connection attempt.

• Quick checks: Use the following command to view supported IKE parameters (output will vary depending on system): ipsec statusall
• Scanning: Nessus plugin ID 8d6444d2 can detect this vulnerability. This is an example only; results may vary.
• Logs and evidence: Examine VPN server logs for IKE negotiation details, specifically looking for Aggressive Mode being used.

ipsec statusall

4. Solution / Remediation Steps

The following steps detail how to fix the issue.

4.1 Preparation

• Ensure you have a rollback plan in case of issues – restore from backup. A change window may be required with appropriate approvals.

4.2 Implementation

1. Step 1: Disable Aggressive Mode if your IKE implementation supports it. This is the preferred solution.
2. Step 2: If disabling Aggressive Mode isn’t possible, avoid using Pre-Shared keys for authentication. Use certificates instead.
3. Step 3: If you must use a Pre-Shared key, ensure it is very strong (long and complex).
4. Step 4: Restrict VPN connections to specific IP addresses if feasible. This limits the attack surface.

4.3 Config or Code Example

Before

#Example IKE configuration (output will vary)
ikev1 aggressive_mode=yes
auth pre-shared-key

After

#Example IKE configuration (output will vary)
ikev1 aggressive_mode=no
auth certificate

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue.

• Practice 1: Least privilege – restrict access to VPN services and configurations.
• Practice 2: Strong authentication – use certificates instead of Pre-Shared keys whenever possible.

4.5 Automation (Optional)

No automation code is provided as configuration varies significantly between vendors.

5. Verification / Validation

Confirm the fix by checking your IKE configuration and verifying that Aggressive Mode is disabled.

• Post-fix check: Run ipsec statusall again to confirm aggressive_mode=no.
• Re-test: Re-run Nessus plugin ID 8d6444d2; it should no longer report the vulnerability.
• Smoke test: Attempt a VPN connection using your standard credentials to ensure connectivity is still working.
• Monitoring: Monitor VPN server logs for any errors related to authentication or IKE negotiation.

ipsec statusall

6. Preventive Measures and Monitoring

Update security baselines and policies to prevent the use of insecure IKE configurations.

• Baselines: Update your security baseline or policy to disallow Aggressive Mode with Pre-Shared keys in IKEv1.
• Pipelines: Implement configuration checks during deployment to ensure compliance with security standards.
• Asset and patch process: Regularly review VPN configurations for vulnerabilities and apply necessary updates.

7. Risks, Side Effects, and Roll Back

Disabling Aggressive Mode may impact compatibility with older clients that only support this mode.

8. References and Resources

Links to resources related to this vulnerability.

• Vendor advisory or bulletin: http://www.nessus.org/u?8d6444d2
• NVD or CVE entry: CVE-2002-1623
• Product or platform documentation relevant to the fix: https://www.ernw.de/download/pskattack.pdf