Index: wp-admin/admin-ajax.php
===================================================================
--- wp-admin/admin-ajax.php	(revision 18448)
+++ wp-admin/admin-ajax.php	(working copy)
@@ -869,7 +869,7 @@
 			die(__('Please provide a custom field value.'));
 		if ( !$meta = get_post_meta_by_id( $mid ) )
 			die('0'); // if meta doesn't exist
-		if ( is_protected_meta( $meta->meta_key, 'post' ) || !current_user_can( 'edit_post_meta', $meta->post_id, $meta->meta_key ) )
+		if ( is_protected_meta( $meta->meta_key, 'post' ) || is_protected_meta( $key, 'post' ) || !current_user_can( 'edit_post_meta', $meta->post_id, $meta->meta_key ) )
 			die('-1');
 		if ( $meta->meta_value != stripslashes($value) || $meta->meta_key != stripslashes($key) ) {
 			if ( !$u = update_meta( $mid, $key, $value ) )
Index: wp-admin/includes/post.php
===================================================================
--- wp-admin/includes/post.php	(revision 18448)
+++ wp-admin/includes/post.php	(working copy)
@@ -667,24 +667,26 @@
 
 	$metakeyselect = isset($_POST['metakeyselect']) ? stripslashes( trim( $_POST['metakeyselect'] ) ) : '';
 	$metakeyinput = isset($_POST['metakeyinput']) ? stripslashes( trim( $_POST['metakeyinput'] ) ) : '';
-	$metavalue = isset($_POST['metavalue']) ? maybe_serialize( stripslashes_deep( $_POST['metavalue'] ) ) : '';
-	if ( is_string($metavalue) )
+	$metavalue = isset($_POST['metavalue']) ? $_POST['metavalue'] : '';
+	if ( is_string( $metavalue ) )
 		$metavalue = trim( $metavalue );
 
-	if ( ('0' === $metavalue || ! empty ( $metavalue ) ) && ((('#NONE#' != $metakeyselect) && !empty ( $metakeyselect) ) || !empty ( $metakeyinput) ) ) {
+	if ( ('0' === $metavalue || ! empty ( $metavalue ) ) && ( ( ( '#NONE#' != $metakeyselect ) && !empty ( $metakeyselect) ) || !empty ( $metakeyinput ) ) ) {
 		// We have a key/value pair. If both the select and the
 		// input for the key have data, the input takes precedence:
 
- 		if ('#NONE#' != $metakeyselect)
+ 		if ( '#NONE#' != $metakeyselect )
 			$metakey = $metakeyselect;
 
-		if ( $metakeyinput)
+		if ( $metakeyinput )
 			$metakey = $metakeyinput; // default
 
 		if ( is_protected_meta( $metakey, 'post' ) || ! current_user_can( 'add_post_meta', $post_ID, $metakey ) )
 			return false;
 
-		return add_post_meta($post_ID, $metakey, $metavalue);
+		$metakey = esc_sql( $metakey );
+
+		return add_post_meta( $post_ID, $metakey, $metavalue );
 	}
 
 	return false;