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> Chapter 25 - Security Profiles > SSL/SSH Inspection

SSL/SSH Inspection

At one point deep inspection was something that was either enabled or not. Now individual deep inspection security profiles can be created depending on the requirements of the policy. Depending on the Inspection Profile, you can:

  • Configure which CA certificate will be used to decrypt the Secure Sockets Layer (SSL) encrypted traffic.
  • Configure which SSL protocols will be inspected.
  • Configure which ports will be associated with which SSL protocols for the purpose of inspection.
  • Configure which websites will be exempt from SSL inspection
  • Configure whether or not to allow invalid SSL certificates.
  • Configure whether or not Secure Shell (SSH) traffic will be inspected.

SSL inspection

Secure Sockets Layer (SSL) content scanning and inspection allows you to apply antivirus scanning, web filtering, FortiGuard Web Filtering, and email filtering to encrypted traffic. To perform SSL content scanning and inspection, the FortiGate unit does the following:

  • intercepts and decrypts HTTPS, IMAPS, POP3S, SMTPS, and FTPS sessions between clients and servers (FortiGate SSL acceleration speeds up decryption)
  • applies content inspection to decrypted content, including:
  • HTTPS, IMAPS, POP3S, and SMTPS Antivirus, DLP, and DLP archiving
  • HTTPS web filtering and FortiGuard web filtering
  • IMAPS, POP3S, and SMTPS email filtering
  • encrypts the sessions and forwards them to their destinations.
FortiGate SSL content scanning and inspection packet flow


Inspection Exemption

When you are using a browser to visit SSL encrypted sites and you are using a certificate that does not match the certificate of the site, you are presented with a warning message and the option of continuing, using the untrusted certificate, or terminating the session. However, there are a number of applications that use SSL encrypted traffic. If the application detects SSL traffic that wasn't signed with a certificate that it trusts it will not allow the traffic. The applications do not give the option to manually indicate that we trust the certificate or the site.

If the option is available, you may choose to import needed SSL certificates into Local Certificates and configure a policy for communication for that application.

The assist in preventing loss of access to these site but still enabling the SSL inspection of the rest of the internet traffic, a method of exempting either Website categories or specific sites has been developed. To exempt a large group of sites the profile can be configure to exempt FortiGuard Categories. There are 3 of these categories preselected due to the high likelihood of issues with associated applications with the type of websites included in these categories.

  • Heath and Wellness
  • Personal Privacy
  • Finance and Banking

Other more specific websites can be added to the exemption list by going to Security Profiles> SSL/SSH Inspection and adding addresses under Exempt from SSL Inspection for the appropriate profile. The adding of addresses is done by selection from a drop down menu.

With the release of FortiOS 5.4.0, some common sense exemptions have been added to the default SSL deep inspection profile so that there will be no interference due to certificate issues. For example: Fortinet, Android, Apple, Skype, and many more.

HTTP Strict Transport Security (HSTS) Protocol

HSTS is a protocol used by Google and other web browsers to prevent man-in-the-middle attacks.

When performing deep inspection, the FortiGate intercepts the https traffic and would send its own self-signed CA certificate to the browser. If the browser is configured to use HSTS connections, it would refuse the FortiGate CA certificate since it is not on the trusted list for Google servers.

To keep the CA certificate from being refused, the HSTS settings should be cleared from the browser. Instructions for this vary between browsers.

Allow Invalid SSL Certificate

This setting was something that used to be part of the Proxy Options, but now that SSL inspection has it’s own configuration setting it is configured with those. It might seem like a straighforward decision that the allowing of invalid SSL certificates must be bad and therefore should not be allowed, but there can be some reasons that should be considered. The issues at hand are the reasons to use a SSL certificate and the reasons that a certificate will be considered invalid.

At a purely technical level, a properly formed certificate will encrypt the data so that it can only be read by the intended parties and not be read by anyone sniffing traffic on the network. For this reason, people will often use self-signed certificates. These self-signed certificates are free and will encrypt the data just as well as those purchased from any of the big vendors of certificates, but if they are not listed as an approved Certificate Authority (CA) the certificates will be considered invalid.

On the other hand, one of the services the vendors provide is verification of identity of those that purchase their certificates. This means that if you see a valid certificate from a site that identified itself as being from “” that you can be reasonably sure that the site does belong to that company and not a false site masquerading as being part of that company.