function wp_embed_handler_youtube( $matches, $attr, $url, $rawattr ) {
	global $wp_embed;
	$embed = $wp_embed->autoembed( "https://youtube.com/watch?v={$matches[2]}" );

	/**
	 * Filters the YoutTube embed output.
	 *
	 * @since 4.0.0
	 *
	 * @see wp_embed_handler_youtube()
	 *
	 * @param string $embed   YouTube embed output.
	 * @param array  $attr    An array of embed attributes.
	 * @param string $url     The original URL that was matched by the regex.
	 * @param array  $rawattr The original unmodified attributes.
	 */
	return apply_filters( 'wp_embed_handler_youtube', $embed, $attr, $url, $rawattr );
}
The software does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.