Index: src/wp-includes/user.php
===================================================================
--- src/wp-includes/user.php	(revision 41829)
+++ src/wp-includes/user.php	(working copy)
@@ -1820,8 +1820,14 @@
 	// Escape data pulled from DB.
 	$user = add_magic_quotes( $user );
 
-	if ( ! empty( $userdata['user_pass'] ) && $userdata['user_pass'] !== $user_obj->user_pass ) {
-		// If password is changing, hash it now
+	if ( empty( $wp_hasher ) ) {
+		require_once( ABSPATH . WPINC . '/class-phpass.php');
+		// By default, use the portable hash from phpass.
+		$wp_hasher = new PasswordHash( 8, true );
+	}
+
+	if ( ! empty( $userdata['user_pass'] ) && false === $wp_hasher->CheckPassword( $userdata['user_pass'], $user_obj->user_pass ) ) {
+		// If password is changing, hash it now.
 		$plaintext_pass = $userdata['user_pass'];
 		$userdata['user_pass'] = wp_hash_password( $userdata['user_pass'] );