This solution was suggested by Raymond Dijkxhoorn on BugTraq:

If you cannot fix it (virtual servers) fast for all your clients you could also try withsomething like this:        RewriteEngine On        RewriteCond %{QUERY_STRING} ^(.*)echr(.*) [OR]        RewriteCond %{QUERY_STRING} ^(.*)esystem(.*)        RewriteRule ^.*$                                -               [F]We had some vhosts where this worked just fine. On our systems we didnt see any validrequest with echr and esystem, just be gentle with it, it works for me, it could workfor you ;)