--- a/wp/wp-trackback.php	Fri Sep 05 18:40:08 2025 +0200
+++ b/wp/wp-trackback.php	Fri Sep 05 18:52:52 2025 +0200
@@ -60,7 +60,14 @@
 
 if ( $charset ) {
 	$charset = str_replace( array( ',', ' ' ), '', strtoupper( trim( $charset ) ) );
-} else {
+
+	// Validate the specified "sender" charset is available on the receiving site.
+	if ( function_exists( 'mb_list_encodings' ) && ! in_array( $charset, mb_list_encodings(), true ) ) {
+		$charset = '';
+	}
+}
+
+if ( ! $charset ) {
 	$charset = 'ASCII, UTF-8, ISO-8859-1, JIS, EUC-JP, SJIS';
 }