Index: src/wp-includes/theme.php
===================================================================
--- src/wp-includes/theme.php	(revision 44703)
+++ src/wp-includes/theme.php	(working copy)
@@ -1081,6 +1081,7 @@
 
 	$width  = absint( $header->width );
 	$height = absint( $header->height );
+	$alt = get_post_meta( $header->attachment_id, '_wp_attachment_image_alt', true );
 
 	$attr = wp_parse_args(
 		$attr,
@@ -1088,7 +1089,7 @@
 			'src'    => $header->url,
 			'width'  => $width,
 			'height' => $height,
-			'alt'    => get_bloginfo( 'name' ),
+			'alt'    => $alt,
 		)
 	);