Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 22136)
+++ wp-includes/functions.php	(working copy)
@@ -245,13 +245,18 @@
  * @return bool False if not serialized and true if it was.
  */
 function is_serialized( $data ) {
+	static $is_overloaded;
+
+	if ( ! isset( $is_overloaded ) )
+		$is_overloaded = ( ( ini_get( 'mbstring.func_overload' ) & 2 ) != 0 ) && function_exists( 'mb_strlen' );
+
 	// if it isn't a string, it isn't serialized
 	if ( ! is_string( $data ) )
 		return false;
 	$data = trim( $data );
  	if ( 'N;' == $data )
 		return true;
-	$length = strlen( $data );
+	$length = ( $is_overloaded ) ? mb_strlen( $data, 'pass' ) : strlen( $data );
 	if ( $length < 4 )
 		return false;
 	if ( ':' !== $data[1] )
@@ -284,11 +289,16 @@
  * @return bool False if not a serialized string, true if it is.
  */
 function is_serialized_string( $data ) {
+	static $is_overloaded;
+
+	if ( ! isset( $is_overloaded ) )
+		$is_overloaded = ( ( ini_get( 'mbstring.func_overload' ) & 2 ) != 0 ) && function_exists( 'mb_strlen' );
+
 	// if it isn't a string, it isn't a serialized string
 	if ( !is_string( $data ) )
 		return false;
 	$data = trim( $data );
-	$length = strlen( $data );
+	$length = ( $is_overloaded ) ? mb_strlen( $data, 'pass' ) : strlen( $data );
 	if ( $length < 4 )
 		return false;
 	elseif ( ':' !== $data[1] )