Can you please be more precise in what the restrictions should be? "UTF-8 names" is not a clear target, as ASCII for instance is a subset of UTF-8.

The restriction is that certain x509 extensions should be encoded as an IA5String, which is a restricted character set(Essentially 0x0-0x80 ASCII). Currently, golang's x509 package performs validation checks on IA5String encoding, but it's not complete. It's still possible to encode several of the Subject Alternative Name extensions ("Domains" "EmailAddresses") as non-IA5Stings. There's also no check on the "URIs" field, but due to URL.String() being called before encoding, it's escaped and therefore won't contain any characters outside of the IA5String set.

One thing to be aware of, the Common Name on an X509 certificate doesn't actually have an IA5String restriction on encoding. You need to use punycode if you're creating an SSL certificate, but RFC 5280 states that "Standard naming attributes, such as common name, employ the DirectoryString type, which supports internationalized names through a variety of language encodings. Conforming implementations MUST support UTF8String and PrintableString."

As far as comparisons go, Certificate.VerifyHostname(host string) will not reject comparisons for non-punycode hostnames in the latest release of go - Go Playground example

However, a recent change (4f9ec2c#diff-14a36701d822b09a804f852a229dfc23R910), was introduced which will result in the rejection of CommonName's with non-ASCII characters in them - caused by validHostname(c.Subject.CommonName)

That being said, if the certificate has a non-ASCII character in the DNSName SAN extension, it will still do the comparison and match. Example below:

c := &x509.Certificate{
        DNSNames: []string{"göögle.com"},
	Subject: pkix.Name{
		CommonName: "göögle.com",
	},
}

err := c.VerifyHostname("göögle.com")
if err == nil {
	fmt.Println("VerifyHostname(göögle.com) should have failed, did not")
} else {
	fmt.Printf("VerifyHostname(göögle.com) failed - %q", err)
}

Change https://golang.org/cl/141837 mentions this issue: crypto/x509: reject UTF-8 names from VerifyHost

Please note that X.509 client certificates contain personal names in the common name (CN) attribute!

Those can be encoded in various ways, one of the more exotic cocktail 🍸of UCS-2/UTF-16/UTF-8 being used in Estonian 🇪🇪 electronic ID: https://eid.eesti.ee/index.php/Authenticating_in_web_applications#Add_user_authentication_to_your_application

Change https://golang.org/cl/235078 mentions this issue: crypto/x509: enforce SAN IA5String encoding restrictions