Setting Up SSL Decryption on Your NGFW: What to Know Before You Start

Encrypted traffic is now the default. Whether users are browsing websites, accessing cloud apps, or syncing data between systems, chances are it’s all wrapped in TLS. That’s great for privacy, but not always great for security visibility. If you can't inspect the traffic, you can't defend against what's hiding inside it.

Next-generation firewalls offer SSL decryption to solve this problem, but configuring it correctly takes more than flipping a switch. It’s a powerful capability that comes with real-world implications for performance, privacy, trust, and policy management. If you are planning to implement it, here’s what you need to know.

Why SSL Decryption Matters

More than 90 percent of modern web traffic is encrypted. That includes malware, command-and-control traffic, data exfiltration, and phishing kits. Without decryption, your firewall is blind to most of what passes through it.

Enabling SSL decryption allows your NGFW to inspect the contents of that encrypted traffic using the same policy engine and threat detection capabilities already in place for unencrypted traffic. This means better detection, better reporting, and stronger enforcement across users and applications.

Types of SSL Decryption

There are two main methods:

• Forward Proxy (Outbound Decryption):Used to inspect traffic from internal users going out to the internet. This is the most common use case and typically involves installing a trusted root certificate on client machines so they accept the firewall as an intermediary.

• Inbound Inspection:Used for inspecting encrypted traffic coming into a public-facing service you host (e.g., a web server or application). Requires importing the private key of the server certificate into the firewall.

  
  

Each has its own configuration steps and use cases, and many environments benefit from running both.

Key Considerations Before Enabling It

• Certificates and Trust:For outbound decryption, clients must trust the firewall’s certificate authority. You’ll need to deploy a root cert across managed devices. Without this, users will see errors or be blocked from access.

• Privacy and Compliance:Not all traffic should be decrypted. Healthcare, finance, and legal applications may carry sensitive or regulated data. Be thoughtful about what gets excluded and ensure your exclusions are documented.

• Performance Impact:Decryption adds CPU overhead. Firewalls not sized for it can experience latency or bottlenecks. Use sizing calculators or vendor guidance to plan capacity.

• Break-Fix and Troubleshooting:Some apps and websites use pinned certificates or do not tolerate decryption. You’ll need to create exceptions and monitor logs to catch and fix issues early.

Best Practices for Implementation

• Start with a monitor-only policy. Identify what would be decrypted, what would break, and which categories need exceptions.

• Roll out in phases, starting with test users or a pilot group.

• Deploy clear user messaging if needed, and make sure help desk staff are briefed on what to expect.

• Maintain a living list of exceptions for services that do not work well with decryption.

• Use categories and groups to build scalable policy logic instead of relying on IP or FQDN alone.

Where It Pays Off

Once SSL decryption is properly in place, you regain critical visibility. You can detect threats inside encrypted traffic, apply URL filtering at a more granular level, and enforce application usage policy with much greater precision. It also improves reporting accuracy and helps correlate user activity with alerts.

For environments with compliance or audit requirements, decrypted logging helps close the visibility gap.