How to Implement and Test SSL Decryption

Outbound SSL Decryption (SSL Forward Proxy)

• In this case, the firewall proxies outbound SSL connections by intercepting outbound SSL requests and generating a certificate on the fly for the site that the user wants to visit.
• The validity date on the PA-generated certificate is taken from the validity date on the real server certificate.
• The issuing authority of the PA-generated certificate is the Palo Alto Networks device.
• If the firewall’s certificate is not part of an existing hierarchy or is not added to a client’s browser cache, then the client receives a warning when browsing to a secure website.
• If the real server certificate has been issued by an authority not trusted by the Palo Alto Networks firewall, then the decryption certificate is using a second “untrusted” Certificate Authority (CA) key to ensure the user is warned of any subsequent man-in-the-middle attacks.

Resolution

To configure SSL decryption:

1. Configure the firewall to handle traffic and place it in the network
2. Make sure the proper Certificate Authority (CA) is on the firewall
3. Configure SSL decryption rules
4. Enable SSL decryption notification page (optional)
5. Commit changes and test decryption

Steps to Configure SSL Decryption

1. Configure the Firewall to Handle Traffic and Place it in the Network

Make sure the Palo Alto Networks firewall is already configured with working interfaces (i.e., Virtual Wire, Layer 2, or Layer 3), Zones, Security Policy, and already passing traffic.

2. Load or Generate a CA Certificate on the Palo Alto Networks Firewall

A Certificate Authority (CA) is required to decrypt traffic properly by generating SSL certificates on the fly. Create a self-signed CA on the firewall or import a Subordinate CA (from your own PKI infrastructure). Select one CA for the Forward Trust Certificate and a different CA for the Forward Untrust Certificate to enable the firewall to decrypt traffic.

NOTE: Because SSL certificate providers such as Entrust, Verisign, Digicert, and GoDaddy do not sell CAs, they are not supported in SSL Decryption.

From the firewall web interface, go to Device > Certificates. Load or generate a certificate for either inbound inspection or outbound (forward proxy) inspection.

Generating a Self-Signed Certificate

Using a Self-Signed Certificate is recommended. For information on generating a Self-Signed Certificate, please review the following Knowledge article: How to Generate a New Self-Signed SSL Certificate.

Generating and Importing a Certificate from Microsoft Certificate Server

1. On the Microsoft Certificate Server for your organization, request an advanced certificate using the certificate template “subordinate CA.” Download the cert.
2. After downloading, export the certificate from the local certificate store. In Internet Explore (IE), access the Internet Options dialog, select the Contenttab, then click the Certificatesbutton. The new certificate can be exported from the personal certificates store. Select Certificate Export Wizard, export the private key, then select the format. Enter a passphrase and a file name and location for the resulting file. The certificate will be in a PFX format (PKCS #12).
3. To extract the certificate, use this openSSL[4] command:
   openssl pkcs12 –in pfxfilename.pfx –out cert.pem –nokeys
4. To extract the key, use this openSSL command:
   openssl pkcs12 –in pfxfilename.pfx –out keyfile.pem -nocerts
5. Import the cert.pem file and keyfile.pem file into the Palo Alto Networks firewall on the Device tab >Certificates screen.
6. In the case of a High Availability (HA) Pair, also load these files into the second Palo Alto Networks firewall, or copy the certificate and key via the High Availability widget on the dashboard.

The "Forward Trust" and "Forward Untrust" certificates:

NOTE: If you're using a self-signed CA, export the public CA certificate from the firewall and install the certificate as a Trusted Root CA on each machine's browser to avoid Untrusted Certificate error messages inside your browser. Network administrators usually use GPO to push out this certificate to each workstation.

Below are some examples of browser errors if the self-signed CA Certificate is not trusted.

Firefox untrusted CA error:

Chrome untrusted CA error:

Internet Explorer untrusted CA error:

3. Configure SSL Decryption Rules

The network administrator determines what needs to be decrypted. A few suggestions for configuring SSL decryption rules:

• Implement rules in a phased approach. Start with specific rules for decryption, and monitor the typical number of SSL connections being decrypted by the device.
• Avoid decrypting the following URL categories, as users may consider this an invasion of privacy:
  • Financial services
  • Health and medicine
  • You can either block or allow connections requiring client authentication via the decryption profile feature introduced in PAN-OS 5.0.

  Here is an example of an outbound rule base following suggestions for decryption:

  4. Enable SSL Decryption Notification Web Page (optional)

  • The user can be notified that their SSL connection will be decrypted using the response page found on the Device tab >Response Pages screen. Click Disabled, check the Enable SSL Opt-out Page option, and click OK.
    

  The default SSL Opt-out page can be exported, edited via an HTML editor, and imported to provide company-specific information:

  5. Test Outbound Decryption

  To test outbound decryption:

  • In the outbound policy, make sure the action is set to alert for any viruses found. Also, enable packet capture on that anti-virus security profile. Commit any changes made.
  • On a PC internal to the firewall, go to www.eicar.org. In the top right corner, you should see this:
    
  • Click “Download anti-malware testfile."
  • In the screen that appears, scroll to the bottom.
  • Download the eicar test virus using HTTP. Any of the these four files will be detected.
  • Go to the Monitor tab >Threat log and then look for the log message that detects the eicar file.
    
  • Click the magnifying glass in the far left column to see the log detail.
    
  • Scroll to the bottom, and look for the field “Decrypted.” The session was not decrypted:
    
  • Go back to the www.eicar.org downloads page. This time use SSL enabled protocol HTTPS to download the test virus.
    
  • Examine the threat logs. The virus should have been detected since the SSL connection was decrypted. A log message that shows eicar was detected in web browsing on port 443 will be visible.
    
  • View the packet capture by clicking the green arrow. (optional)
    
  • To the left of that log entry, click the magnifying glass. Scroll to the bottom. Under Flags, check to see if the “Decrypted” box is checked:
    

  The virus was successfully detected in an SSL-encrypted session.

  To test the “no-decrypt” rule:

  • First, determine what URLs fall into financial services, health and medicine categories, and any categories that decryption is not enabled. Use Palo Alto Networks URL Filtering - Test A Site and enter a URL to identify the category.
  • Once websites are classified into categories and will not be decrypted are found, use a browser to go to those websites using HTTPS. There should be no certificate error when going to those sites. The web pages will be displayed properly. Traffic logs will show the sessions where application SSL traverses port 443, as expected.

  To Test Inbound Decryption:

  • Examine the traffic logs dated before enabling SSL for inbound decryption on the firewall. Look at traffic targeted for the internal servers. In those logs, the application detected should be “ssl" going over port 443.
  • From a machine outside the network, connect via SSL to a server in the DMZ. There will be no certificate errors, as the connection is not being proxied—just inspected.
  • Examine the logs for this inbound connection. The applications will not be “ssl" but the actual applications found inside the SSL tunnel. Click the magnifying glass icon in those log entries to confirm decrypted connections.
    

  Helpful CLI Commands

  • To see how many existing SSL decryption sessions are going through the device, use this CLI command:

  > show session all filter ssl-decrypt yes count yes

  • To see the active sessions that have been decrypted, use this CLI command:

  > show session all filter ssl-decrypt yes state active

  • To check if there are any sessions hitting the limit of the device, use this CLI command:

  > show counter global name proxy_flow_alloc_failure

  • To view the SSL decryption certificate, use this CLI command:

  > show system setting ssl-decrypt certificate Certificates for Global SSL Decryption CERT global trusted ssl-decryption x509 certificate version 2 cert algorithm 4 valid 150310210236Z -- 210522210236Z cert pki 1 subject: 172.16.77.1 issuer: 172.16.77.1 serial number(9) 00 b6 96 7e c9 99 1f a8 f7 . ~. . rsa key size 2048 siglen 2048 basic constraints extension CA 1 global untrusted ssl-decryption x509 certificate version 2 cert algorithm 4 valid 150310210236Z -- 210522210236Z cert pki 1 subject: 172.16.77.1 issuer: 172.16.77.1 serial number(9) 00 b6 96 7e c9 99 1f a8 f7 . ~. . rsa key size 2048 siglen 2048 basic constraints extension CA 1

  • To view SSL decryption settings, use this CLI command:

  > show system setting ssl-decrypt setting vsys : vsys1 Forward Proxy Ready : yes Inbound Proxy Ready : no Disable ssl : no Disable ssl-decrypt : no Notify user : no Proxy for URL : no Wait for URL : no Block revoked Cert : yes Block timeout Cert : no Block unknown Cert : no Cert Status Query Timeout : 5 URL Category Query Timeout : 5 Fwd proxy server cert's key size: 0 Use Cert Cache : yes Verify CRL : no Verify OCSP : no CRL Status receive Timeout : 5 OCSP Status receive Timeout : 5 Block unknown Cert : no

  Additional Information

  • Documentation - Configure SSL Forward Proxy
  • Documentation - Configure SSL Inbound Inspection
  • SSL Decryption Quick Reference - Resources
  • SSL Decryption Not Working Due to Unsupported Cipher Suites
  • Limitations and Recommendations While Implementing SSL Decryption
  • How to Identify Root Cause for SSL Decryption Failure Issues