Azure Functions - Configure client certificate authentication

Question

Do functions support authorizing access to a Function by using client certificates, in a consumption plan? Something similar to the approach described here? Basically, I'm looking for the Functions runtime to immediately reject connection requests if the caller does not present a valid client certificate, without me having to implement that authorization routine in the code.

Answer 1

Here's the code I came up with, note: this is for Azure Functions v1, when req is an HttpRequestMessage

Caller:

X509Certificate2 clientCert = req.GetClientCertificate();

if (!IsValidClientCertificate(clientCert))
{
    return req.CreateErrorResponse(HttpStatusCode.Unauthorized, "A valid client certificate is not found");
}

For Azure Functions v2, you can get the client certificate from the HttpRequest using req.HttpContext.Connection.ClientCertificate

Basic validation function:

static bool IsValidClientCertificate(X509Certificate2 clientCert)
{
    // check the cert's thumbprint against expected thumbprint
    if (clientCert.Thumbprint != "<expected thumprint>"
    { 
        return false;
    }

    // check that we're within the cert's validity period
    if (DateTime.Now > clientCert.NotAfter || DateTime.Now < clientCert.NotBefore)
    {
        return false;
    }

    // optionally check cert chaining validity
    // if(!clientCert.Verify()) { return false; }
}