Index: wp-includes/comment-template.php
===================================================================
--- wp-includes/comment-template.php	(revision 26836)
+++ wp-includes/comment-template.php	(working copy)
@@ -2043,7 +2043,7 @@
 					do_action( 'comment_form_must_log_in_after' );
 					?>
 				<?php else : ?>
-					<form action="<?php echo site_url( '/wp-comments-post.php' ); ?>" method="post" id="<?php echo esc_attr( $args['id_form'] ); ?>" class="comment-form"<?php echo $html5 ? ' novalidate' : ''; ?>>
+					<form action="<?php echo apply_filters( 'comment_form_action', site_url( '/wp-comments-post.php' ) ); ?>" method="post" id="<?php echo esc_attr( $args['id_form'] ); ?>" class="comment-form"<?php echo $html5 ? ' novalidate' : ''; ?>>
 						<?php
 						/**
 						 * Fires at the top of the comment form, inside the <form> tag.