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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 Certificate Authority (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 straightforward decision that the allowing of invalid SSL certificates must be bad and, therefore, should not be allowed. However, there can be some reasons that applying this feature should be considered.

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 securely as a purchased certificate. The self-signed certificates, however, are not likely to recognized by the CA certificate store so will be considered by any checks against that store as 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.

You can allow invalid SSL certificates by going to Security Profiles> SSL/SSH Inspection, selecting the appropriate profile, and enabling Allow Invalid SSL Certificates.

During the SSL handshake, a number of checks are made to verify the validity of the certificate.

One source of the checks, is against a CA certificate store inside FortiOS. This is the same CA bundle used by the browser Mozilla Firefox. Update to the store are:

  • made with each new version of FortiOS
  • available through internal FGD
  • Possible with some builds via FTP

Details of the CA certificate store can be found at:

The following checks are made for validity:

Validity Check Description

One of the things being checked against the CA bundle is the certificate signature. These signatures are generated via directly signing by the CA's private key.

Expiration date

All certificates have an expiry date. The date, based on the devices clock/calendar is compared to the expiry date of the certificate.

Revoked list

Periodically, certificates are revoked. If a certificate has been revoked it is put on a list. Whenever a certificate is being verified, it is checked against this list.

Self signed certificate

In the case of self-signed certificates, the IPS engine and proxy have different handling. IPS engine will keep and use the certificate self-signed certificate, but the public key will be replaced so that SSL inspection can take place. The proxy engine will re-sign the certificate with the untrusted CA certificate. The mechanics are similar but the net effect for the user is similar. The user will get warnings from browsers. The users can choose to remember the self-signed certificate in some browsers, but cannot do the same thing with the certificate re-signed with the untrusted CA.

Intermediate CA with a weak hash algorithm, such as MD5, SHA1

Some browsers like Chrome or Firefox will give a warning because of a weak signature algorithm (visit to test).

In the IPS Engine, in order to convey the weak intermediate CA back to client, signature hash algorithm is downgraded in the re-signed server certificate to the weakest algorithm used in the original certificate chain.

In the Proxy Engine - In the case of a weak signature algorithm, the Proxy engine will treat the connection as untrusted, and re-sign the server certificate with the untrusted CA. The final user experience is different. Instead of a warning like "NET::ERR_CERT_WEAK_SIGNATURE_ALGORITHM” that you would get in Chrome, you will get a warning that the certificate couldn’t be verified (because of the signing CA is not trusted or imported into the user’s web browser).