mysql: don't enable cleartext passwords on unverified connections

mysql/mysql.go

@@ -79,7 +79,13 @@ func ConfigFromURL(u *url.URL) (cfg *mysql.Config, err error) {
 	cfg.Passwd, _ = u.User.Password()
 	cfg.DBName = dbName
 	if _, ok := u.Query()["allowCleartextPasswords"]; !ok {
-		cfg.AllowCleartextPasswords = true
+		// The cleartext auth plugin hands the password to whatever answers on
+		// the database address, so it is only safe once the server's identity
+		// has been verified. Enable it by default only when the connection is
+		// TLS-protected with certificate verification and no plaintext
+		// fallback; otherwise applications must opt in explicitly with
+		// "allowCleartextPasswords=true".
+		cfg.AllowCleartextPasswords = cfg.TLS != nil && !cfg.TLS.InsecureSkipVerify && !cfg.AllowFallbackToPlaintext
 	}
 	cfg.AllowNativePasswords = true
 	return cfg, nil

mysql/mysql_test.go

@@ -99,10 +99,30 @@ func TestConfigFromURLCleartextPasswords(t *testing.T) {
 		want bool
 	}{
 		{
-			name: "default",
+			name: "default, no TLS",
 			url:  "mysql://user:password@localhost/db",
+			want: false,
+		},
+		{
+			name: "default, no TLS, other query params present",
+			url:  "mysql://user:password@localhost/db?timeout=5s",
+			want: false,
+		},
+		{
+			name: "default, TLS with verification",
+			url:  "mysql://user:password@localhost/db?tls=true",
 			want: true,
 		},
+		{
+			name: "default, TLS without verification",
+			url:  "mysql://user:password@localhost/db?tls=skip-verify",
+			want: false,
+		},
+		{
+			name: "default, TLS with plaintext fallback",
+			url:  "mysql://user:password@localhost/db?tls=preferred",
+			want: false,
+		},
 		{
 			name: "explicit false",
 			url:  "mysql://user:password@localhost/db?allowCleartextPasswords=false",
@@ -113,6 +133,11 @@ func TestConfigFromURLCleartextPasswords(t *testing.T) {
 			url:  "mysql://user:password@localhost/db?allowCleartextPasswords=true",
 			want: true,
 		},
+		{
+			name: "explicit true without TLS",
+			url:  "mysql://user:password@localhost/db?allowCleartextPasswords=true&tls=false",
+			want: true,
+		},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			u, err := url.Parse(tc.url)