Improve tls certificate verification

internal/mqtt/tls.go

@@ -4,6 +4,8 @@ import (
 	"MQTTLogger/config"
 	"crypto/tls"
 	"crypto/x509"
+	"fmt"
+	"net/url"
 	"os"
 
 	"go.uber.org/zap"
@@ -25,10 +27,24 @@ func NewTLSConfig(logger *zap.Logger, config *config.Config) *tls.Config {
 
 	return &tls.Config{
 		RootCAs:            certpool,
-		// We use the provided cert not the one server sends.
-		ClientAuth:         tls.NoClientCert,
-		ClientCAs:          nil,
-		InsecureSkipVerify: true, // I know
+		InsecureSkipVerify: true,
 		Certificates:       nil,
+		VerifyPeerCertificate: func(rawCerts [][]byte, _ [][]*x509.Certificate) error {
+			cert, err := x509.ParseCertificate(rawCerts[0])
+			if err != nil {
+				return fmt.Errorf("failed to parse certificate: %w", err)
+			}
+
+			opts := x509.VerifyOptions{Roots: certpool}
+			if _, err := cert.Verify(opts); err != nil {
+				return fmt.Errorf("failed to verify chain: %w", err)
+			}
+
+			expectedCN, _ := url.Parse(config.URI)
+			if cert.Subject.CommonName != expectedCN.Hostname() {
+				return fmt.Errorf("unexpected CN, expected %s but got %s", expectedCN.Host, cert.Subject.CommonName)
+			}
+			return nil
+		},
 	}
 }