Use SSL Inbound Inspection to decrypt and inspect inbound SSL/TLS traffic from a client to a targeted network server (any server you have the certificate for and can import it onto the firewall) and block suspicious sessions. For example, suppose a malicious actor wants to exploit a known vulnerability in your web server. Inbound SSL/TLS decryption provides visibility into the traffic, allowing the firewall to respond to the threat proactively.

The way the firewall performs SSL Inbound Inspection depends on the type of key exchange in use—Rivest, Shamir, Adleman (RSA) or Perfect Forward Secrecy (PFS). The Diffie-Hellman exchange (DHE) and Elliptic Curve Diffie-Hellman exchange (ECDHE) algorithms provide PFS. For the RSA key exchange, the firewall performs SSL Inbound Inspection without terminating the connection. As the encrypted session flows through the firewall, the firewall transparently makes a copy of it and decrypts it so that the firewall can apply the appropriate policy to the traffic. In other words, the firewall passively observes and decrypts inbound traffic using the server’s private key without being detected by the client and server.

For PFS key exchange algorithms (DHE or ECDHE), the firewall acts as a man-in-the-middle proxy between the external client and the internal server. Because PFS methods generate a new key for every session, the firewall can’t simply copy and decrypt the inbound SSL flow as it passes through and must act as a proxy device.

On the firewall, you must install the certificate and private key for each server for which you want to perform SSL Inbound Inspection. The TLS versions and key exchange algorithms that your web server support determine how you should install the server certificate on the firewall. If your web server supports TLS 1.2 and PFS key exchange algorithms and your end-entity (leaf) certificate is signed by intermediate certificates, we recommend uploading a certificate chain (a single file) to the firewall. Uploading the chain avoids client-side server certificate authentication issues.

The firewall handles TLS 1.3 connections differently than TLS 1.2 connections. During TLS 1.3 handshakes, the firewall sends the client the same certificate or certificate chain that it receives from the server. As a result, uploading the server certificate and private key to the firewall is sufficient if you correctly set up your web server. For example, if your server’s leaf certificate is signed by intermediate certificates, the chain of certificates needs to be installed on the server to avoid client-side server authentication issues.

If your web server supports only TLS 1.2 and the RSA key exchange algorithm, you can upload the server certificate and private key alone because the connection is transparent. As with TLS 1.3 connections, the certificate or certificate chain configured on the web server is the same sent to the client.

The following figure shows how SSL Inbound Inspection works when the key exchange algorithm is RSA. When a PFS key exchange algorithm is in use, the firewall functions as a proxy (creates a secure session between the client and the firewall and another secure session between the firewall and the server) and generates a new session key for each session.

To protect an internal server, follow the steps to configure SSL Inbound Inspection policy rules.