Cannot access Outlook in Chrome.

Question

An application is stopping Chrome from safely connecting to this site

"Fortinet" wasn’t installed properly on your computer or network. Ask your IT administrator to resolve this issue.

net::ERR_CERT_AUTHORITY_INVALID

Answer 1

Hi Bradley,

Do you have any VPN installed?

If no VPN is in use, a Fortinet firewall on the network—or upstream—may be blocking the connection or performing deep SSL inspection on the traffic.

I’m eager to hear back and work together to find a solution.

Answer 2

The error indicates that Chrome does not trust the SSL certificate being presented for Outlook because the issuing certificate authority (CA) is not trusted, and Fortinet (likely a web filter or proxy) is intercepting HTTPS traffic but is not installed or configured correctly.

To resolve this, the certificate chain used by Fortinet must be trusted on the device, or the interception must be fixed/removed by the administrator. The relevant guidance from the context:

1. When a proxy or security device presents its own certificate, Outlook/Chrome can show errors like net::ERR_CERT_AUTHORITY_INVALID if the root CA is not trusted. This is the same class of problem described as an SSL certificate trust failure.
2. The fix is to ensure the root certificate for the device (Fortinet in this case) is installed in the Trusted Root Certification Authorities store on the client, or to use a certificate that chains to a trusted public CA.
3. If the certificate is self-signed or from a private CA and not trusted by the environment, Office 365/Outlook access will fail until the trust issue is corrected.

Action steps for the IT administrator (since the message explicitly says to contact IT):

1. Examine the certificate being presented when browsing to Outlook through Chrome:
   • Open Chrome and browse to the Outlook URL.
   • View the certificate details and check:
     • Subject: matches the site name being accessed.
     • Issuer: which CA issued it (likely a Fortinet or internal CA).
     • Valid to: certificate is not expired.
2. Confirm whether the issuing CA is trusted:
   • If the certificate is from a private/enterprise CA (including Fortinet’s own CA), install the corresponding root certificate into the Trusted Root Certification Authorities store on the affected machine(s).
   • If the certificate is invalid, revoked, or has the wrong name, replace it with a valid certificate whose subject/SAN matches the Outlook/OWA URL.
3. If a self-signed or untrusted certificate is being used for SSL interception, replace it or disable interception for Office 365/Outlook endpoints so that the browser sees the original, trusted Microsoft certificate.
4. After fixing the certificate trust:
   • Close all Chrome windows.
   • Clear browser cache if needed.
   • Reopen Chrome and test Outlook access again.

If this is a managed or corporate environment, the end user should not attempt to bypass the warning; instead, the IT administrator must correct the Fortinet installation and certificate trust so that Chrome can establish a secure, trusted connection.

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References:

• SSL Certificate Trust Failure
• Error when Outlook tries to connect to a server by using an RPC connection or an HTTPS connection: There is a problem with the proxy server's security certificate