Index: wp-includes/post-template.php
===================================================================
--- wp-includes/post-template.php	(revision 19898)
+++ wp-includes/post-template.php	(working copy)
@@ -292,7 +292,7 @@
  */
 function post_class( $class = '', $post_id = null ) {
 	// Separates classes with a single space, collates classes for post DIV
-	echo 'class="' . join( ' ', get_post_class( $class, $post_id ) ) . '"';
+	echo 'class="' . esc_attr( join( ' ', get_post_class( $class, $post_id ) ) ) . '"';
 }
 
 /**
@@ -384,7 +384,7 @@
  */
 function body_class( $class = '' ) {
 	// Separates classes with a single space, collates classes for body element
-	echo 'class="' . join( ' ', get_body_class( $class ) ) . '"';
+	echo 'class="' . esc_attr( join( ' ', get_body_class( $class ) ) ) . '"';
 }
 
 /**