Develop a PKI Rollout Plan

Ensure that all of your network devices have valid SSL Forward Trust certificates before rolling out decryption to avoid unnecessary certificate warnings and the resulting user calls to tech support.
Plan how to roll out your public key infrastructure (PKI). Network devices need an SSL Forward Trust CA certificate for trusted sites and an SSL Forward Untrust CA certificate for untrusted sites. Generate separate Forward Trust and Forward Untrust certificates (do not sign the Forward Untrust certificate with the Enterprise Root CA because you want the Untrust certificate to warn users that they are trying to access potentially unsafe sites). Palo Alto Networks next-generation firewalls have two methods of generating CA certificates for SSL decryption:
  • Generate the SSL CA certificates from your Enterprise Root CA as subordinate certificates
    —If you have an existing Enterprise PKI, this is the best practice. Generating a subordinate certificate from your Enterprise Root CA makes the rollout easier and smoother because network devices already trust the Enterprise Root CA, so you avoid any certificate issues when you begin the deployment phase. If you don’t have an Enterprise Root CA, consider getting one.
  • Generate a self-signed Root CA certificate on the firewall and create subordinate CA certificates on that firewall
    —If you don’t have an Enterprise Root CA, this method provides a self-signed Root CA certificate and the subordinate Forward Trust and Untrust CA certificates. With this method, you need to install the self-signed certificates on all of your network devices so that those devices recognize the firewall’s self-signed certificates. Because the certificates must be deployed to all devices, this method is better for small deployments and proof-of-concept (POC) trials than for large deployments.
Do not export the Forward Untrust certificate to the Certificate Trust Lists of your network devices! This is critical because installing the Untrust certificate in the Trust List results in devices trusting websites that the firewall does not trust. In addition, users won’t see certificate warnings for untrusted sites, so they won’t know the sites are untrusted and may access those sites, which could expose your network to threats.
Regardless of whether you generate Forward Trust certificates from your Enterprise Root CA or use a self-signed certificate generated on the firewall, generate a separate subordinate Forward Trust CA certificate for each firewall. The flexibility of using separate subordinate CAs enables you to revoke one certificate when you decommission a device (or device pair) without affecting the rest of the deployment and reduces the impact in any situation in which you need to revoke a certificate. Separate Forward Trust CAs on each firewall also helps troubleshoot issues because the CA error message the user sees includes information about the firewall the traffic is traversing. If you use the same Forward Trust CA on every firewall, you lose the granularity of that information.
There is no benefit to using different Forward Untrust certificates on different firewalls, so you can use the same Forward Untrust certificate on all firewalls. If you need additional security for your private keys, consider storing them on an HSM.
You may need to make special accommodations for guest users. If guest users don’t need access to your corporate network, don’t allow access, and then you won’t have to decrypt their traffic or create infrastructure to support guest access. If you need to support guest users, discuss with your legal department whether you can decrypt guest traffic.
If you can decrypt guest traffic, treat guests similarly to the way you treat BYOD devices. Decrypt guest traffic and subject it to the same Security policy that you apply to other network traffic. Do this by redirecting guest users through a captive portal, instruct them how to download and install the CA certificate, and clearly notify users that their traffic will be decrypted. Include the process in your company’s privacy and computer usage policy. In addition, restrict guest traffic to only the areas guests need to access.
If you can’t decrypt guest traffic for legal reasons, then isolate guest traffic and prevent it from moving laterally in your network:
  • Create a separate zone for guests and restrict guest access to that zone. To prevent lateral movement, don’t allow guest access to other zones.
  • Allow only sanctioned applications, use URL filtering to prevent access to risky URL categories, and apply the best practice Security profiles.
  • Apply a No Decrypt decryption policy and profile to prevent guests from accessing websites with unknown or expired CAs.
All employees, contractors, partners, and other users should use your normal corporate infrastructure and you should decrypt and inspect their traffic.

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