diff --git a/wp-includes/functions.php b/wp-includes/functions.php
index ea6ec03372..2a4d860166 100644
--- a/wp-includes/functions.php
+++ b/wp-includes/functions.php
@@ -5436,6 +5436,9 @@ function iis7_supports_permalinks() {
  * @return int 0 means nothing is wrong, greater than 0 means something was wrong.
  */
 function validate_file( $file, $allowed_files = array() ) {
+	// Normalize path for Windows servers
+	$file = wp_normalize_path( $file );
+
 	// `../` on its own is not allowed:
 	if ( '../' === $file ) {
 		return 1;