From 564ec4867bd867ccf37d149243d016abfa5a857c Mon Sep 17 00:00:00 2001
From: Filippo Valsorda <filippo@golang.org>
Date: Fri, 30 Oct 2020 16:12:13 +0100
Subject: [PATCH] crypto/tls: don't use CN in BuildNameToCertificate if SANs
 are present

Change-Id: I18d5b9fc392a6a52fbdd240254d6d9db838073a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/266540
Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
---
 src/crypto/tls/common.go | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/crypto/tls/common.go b/src/crypto/tls/common.go
index 66d2c005a7..86dc0dd3b2 100644
--- a/src/crypto/tls/common.go
+++ b/src/crypto/tls/common.go
@@ -1263,7 +1263,9 @@ func (c *Config) BuildNameToCertificate() {
 		if err != nil {
 			continue
 		}
-		if len(x509Cert.Subject.CommonName) > 0 {
+		// If SANs are *not* present, some clients will consider the certificate
+		// valid for the name in the Common Name.
+		if x509Cert.Subject.CommonName != "" && len(x509Cert.DNSNames) == 0 {
 			c.NameToCertificate[x509Cert.Subject.CommonName] = cert
 		}
 		for _, san := range x509Cert.DNSNames {