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Re: CERT: Vulnerability in web redirectors
From: "Kurt Seifried" <kurt () seifried org>
Date: Fri, 21 Mar 2003 19:59:38 -0800

Like hack4life stated this is a pretty well known problem. Solving it isn't
to hard. Legitimate clients will send an HTTP referer, thus for some
unsuspecting user who clicks on a redirect link in an email they will send a
blank referer, or off a website that referer. So simply add some logic in
your script to check the referer, a quick and dirty in php for example (yes,
this code is horrible and ugly, but it's a reasonably clear example):

<TITLE>You are now leaving example.org</TITLE>
if (preg_match ("/example\.org\//i", $_SERVER["HTTP_REFERER"] )) {

  print "<META http-equiv=\"refresh\" content=\"1;URL=";
  print "\">";
  print "</HEAD>";
  print "<BODY bgcolor=\"#ffffff\">";
  print "<P>[1] Redirecting you to:";
  print "<A HREF=\"";
  print "\">";
  print "</A>, if it doesn't work click on the link.</P>";
  print "<P>";
  print "</P>";
} else {
  print "</HEAD>";
  print "<BODY bgcolor=\"#ffffff\">";
  print "<P>Someone has tried to use this site's web redirector in an
attempt to make a link appear legitimate.</P>";

Obviously this is not bullet proof, anyone who hacks their client to send
fake referer's can get around it, but then we aren't trying to block that
kind of person, we're simply trying to prevent widespread abuse by a

Kurt Seifried, kurt () seifried org
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