Technical Note: Google HSTS servers.
Description
HSTS (HTTP Strict Transport Security) is a protocol used initially by Google to prevent Man in the Middle attacks.
When doing deep inspection, the FortiGate intercepts the https traffic and it would give its own Self-Signed CA certificate to the Browsers. If the Browser is compliant to use HSTS connections, then it would refuse the FortiGate CA certificate as the certificate is not in the Trusted list for Google servers.
When doing deep inspection, the FortiGate intercepts the https traffic and it would give its own Self-Signed CA certificate to the Browsers. If the Browser is compliant to use HSTS connections, then it would refuse the FortiGate CA certificate as the certificate is not in the Trusted list for Google servers.
Solution
Verify if HSTS is enabled in the browser. If it is enabled then the HSTS settings on the browser should be cleared.
There are many published articles on clearing these browser settings, for example, https://appuals.com/how-to-clear-or-disable-hsts-for-chrome-firefox-and-internet-explorer/
There are many published articles on clearing these browser settings, for example, https://appuals.com/how-to-clear-or-disable-hsts-for-chrome-firefox-and-internet-explorer/