diff --git a/src/wp-includes/fonts.php b/src/wp-includes/fonts.php
index 953898136300e..9c928c9b81cee 100644
--- a/src/wp-includes/fonts.php
+++ b/src/wp-includes/fonts.php
@@ -118,6 +118,16 @@ function wp_unregister_font_collection( string $slug ) {
  * }
  */
 function wp_get_font_dir( $defaults = array() ) {
+	/*
+	 * Bail if the 'font_dir' filter is already running.
+	 *
+	 * This avoids an infinite loop that can occur
+	 * if wp_upload_dir() is called inside a 'font_dir' callback.
+	 */
+	if ( doing_filter( 'font_dir' ) ) {
+		return $defaults;
+	}
+
 	$site_path = '';
 	if ( is_multisite() && ! ( is_main_network() && is_main_site() ) ) {
 		$site_path = '/sites/' . get_current_blog_id();
@@ -140,7 +150,7 @@ function wp_get_font_dir( $defaults = array() ) {
 	 *
 	 * @param array $defaults The original fonts directory data.
 	 */
-	return apply_filters( 'font_dir', $defaults );
+	return doing_filter( 'font_dir' ) ? $defaults : apply_filters( 'font_dir', $defaults );
 }
 
 /**