Custom root trust is a security practice where an application or system is configured to trust only a specific set of root certificates. This approach restricts the application’s trust to a limited, predefined list of Certificate Authorities, enhancing security by preventing reliance on external or unverified certificates. The practise is also use in a corporate setting to limit trust to an internal root certificate.

Custom root trust is similar to, but differs from Certificate Pinning, where Certificate Pinning is the act of trusting a specific certificate and custom root trust is trusting all certificates issued by the trusted root certificate.

Implementing custom root trust has been pretty straight forward with HttpClient ever since .NET 5.

X509Certificate2 trustedRoot = new X509Certificate2("corp-root-ca.pem");

HttpClientHandler handler = new()
    ServerCertificateCustomValidationCallback = (_, certificate, chain, errors) =>
        // If errors are present, but they are not Remote Certificate Chain Errors, fail the validation.
        if ((errors & ~SslPolicyErrors.RemoteCertificateChainErrors) != 0)
            return false;

        // Add intermediate certificates from the server's chain for the verification process.
        foreach (X509ChainElement intermediateElements in chain.ChainElements.Skip(1))

        // Clear any potential existing trusted root certificates.
        // Enforce Custom Root Trust.
        chain.ChainPolicy.TrustMode = X509ChainTrustMode.CustomRootTrust;
        // Add desired root certificate.
        // Verify the validity of the chain of trust and the server's certificate
        return chain.Build(certificate);
HttpClient client = new(handler);
HttpResponseMessage response = await client.GetAsync("https://intranet.corp.local");