Microsoft Entra Internet Access is a powerful cloud-based Secure Web Gateway (SWG) feature within the Entra Global Secure Access (GSA) Security Service Edge (SSE) solution. Entra Internet Access provides Zero Trust, identity-aware access to internet resources, private web-based applications, and Microsoft 365, with full integration with Entra Conditional Access.
TLS Inspection
Entra Internet Access includes an optional TLS Inspection feature that allows the GSA client to decrypt HTTPS traffic, inspect for threats, identify policy violations, and enforce Data Loss Prevention (DLP) policies. Importantly, enabling TLS inspection for GSA allows administrators to apply prompt injection protection policies to control the usage of generative AI applications.
TLS Inspection Certificate
Before enabling TLS inspection for Entra Internet Access, administrators must first create a TLS inspection certificate. This certificate must be signed by a trusted certification authority (CA). The process is simple and straightforward, and well-documented here.
Invalid Certificate Error
After enabling Entra Internet Access TLS inspection, administrators may find that all websites subject to TLS inspection are inaccessible. The browser displays the following error message:
Your connection isn’t private
Attackers might be trying to steal your information from <website> (for example, passwords, messages, or credit cards.)
NET:ERR_CERT_INVALID
Clicking on the Advanced button shows the following additional information:
<website> uses encryption to protect your information. When Microsoft Edge tried to connect to <website> this time, the website sent back unusual and incorrect credentials. This may happen when an attacker is trying to pretend to be <website>, or a Wi-Fi sign-in screen has interrupted the connection. Your information is still secure because Microsoft Edge stopped the connection before any data was exchanged.
You can’t visit <website> right now because the website sent scrambled credentials that Microsoft Edge can’t process. Network errors and attacks are usually temporary, so this page will probably work later.
Root Cause (Pun Intended!)
This issue can be caused by restrictions placed on the root CA. Specifically, if the root CA certificate includes a policy that restricts the CA path length (the number of subordinate CAs allowed downstream), the Microsoft Global Secure Access Intermediate CA, which issues certificates for TLS-inspected websites, cannot be validated successfully.
In this example, the root CA certificate includes a basic constraint that defines a maximum of 1 intermediate CA in the chain. Crucially, the extension is marked as Critical, so it must be enforced.
Because the root CA enforces a path length constraint of 1, the TLS inspection subordinate CA can exist beneath it, but no additional subordinate CA certificates are permitted. As a result, the Microsoft Global Secure Access Intermediate CA exceeds the allowed chain depth, causing certificate validation to fail.
Resolution
The fix for this issue is simple, yet complex. The root CA certificate must be renewed, this time without enforcing the CA path length policy. To do this, open an elevated command window on the root CA and run the following command.
Important: If your CA hierarchy uses CAPolicy.inf to define the CAPathLength setting, update the file before renewing the CA certificate.
Next, restart the CA service for the change to take effect.
Finally, renew the CA certificate.
Restart the CA service once more for the change to take effect.
Once complete, distribute the new root CA certificate to Active Directory and to Intune-managed endpoints using a Trusted Certificate device configuration policy.
Finally, configure a new Entra TLS inspection certificate in the Entra admin center to replace the old one, signed with the updated root CA certificate. Once the certificate has been uploaded, ensure it is enabled.
Important: Renewing a root CA certificate can be highly disruptive. Proceed with caution in production environments. Ensure that all enterprise assets receive the new root CA certificate in a timely manner. Alternatively, to reduce the chance of disruption, consider deploying a new root CA dedicated to Entra TLS inspection.
Result
Once these changes are made, the certificate chain will allow the Microsoft Global Secure Access Intermediate CA to exist beneath the TLS inspection CA, resulting in a valid certificate chain for TLS-inspected websites. Browsers will once again trust the dynamically generated certificates, eliminating the ERR_CERT_INVALID error.
The following certificate chain shows the corrected configuration after renewing the root CA certificate and recreating the TLS inspection certificate.
Summary
Entra Internet Access TLS inspection relies on a certificate chain that includes the Microsoft Global Secure Access Intermediate CA. If the root CA that signs the TLS inspection certificate enforces a restrictive path length constraint, certificate validation can fail, causing browsers to display ERR_CERT_INVALID errors for all TLS-inspected websites. Reviewing the certificate chain and understanding how basic constraints affect subordinate CAs can help quickly identify and resolve this issue. When deploying TLS inspection, ensure that CA hierarchy restrictions are compatible with this deployment scenario. Consider using a dedicated PKI hierarchy to minimize operational impact.
Additional Information
Tutorial: Enable Entra Internet Access TLS Inspection
Protect Enterprise Generative AI Applications with Prompt Injection Protection
