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TUCoPS :: Web :: Apps :: mailfile.htm

Mail File CGI POST security hazard

    Mail File POST


    MailFile v 1.10 by Oatmeal-Studios


    Dirk  Brockhausen  found  following.   This  Perl script enables a
    site's visitor  to have  a given  file dispatched  to a  specified
    email address.  The visitor is required to select the file from  a
    given list and to enter his or her email address.

    The data will then be dispatched  by a POST command to the  target

    In contrast to a GET command, a POST command will not display  the
    data  in  the  referenced  URL.   For  example,  if  you search in
    AltaVista for the phrase "ip blocker", the URL generated will look
    like this:

    The query script is  called with GET, which  is why the phrase  is
    displayed in the URL and can even be modified there.

    As the MailFile script uses a  POST command, the file name is  not
    open  to  manipulation  on  the  URL  level.   But this is a false
    security!   Indeed, this  script is  quite vulnerable  to a fairly
    simple attack.

    E.g. an attacker could  install the entry form  on his or her  own
    web site and modify the file name accordingly.  To rule this  out,
    the script will  check the "Referer"  variable.  For  security and
    bandwidth  economy  reasons  it  will  only  permit calls from the
    domain it actually resides on.  Or so it seems.

    Indeed, an  attacker can  manipulate not  only the  UserAgent data
    but the "Referer" variable as well.  It takes only a few lines  of
    Perl to send the required data (variables) to the MailFile script.

    Schematic Code:

        use HTTP::Request::Common;
        use LWP::UserAgent;
        $ua = LWP::UserAgent->new;
        $res = $ua->request(POST 'http://domain/mailfile.cgi',
                            [real_name => 'value1',
                             email => 'value2',
                             filename => 'value3',

    This code can  actually be used  against all cgi  scripts based on
    the POST  method.   In the  case of  the MailFile script discussed
    here, we could even skip sending a Referer because the implemented
    check routine  will simply  regard the  Referrer as  valid if  the
    referer variable has not been set.

    Again,  this  method  allows  for  systemwide opening of all files
    holding read permissions for "others" - a major security hazard!


    For security reasons, this script should not be implemented.

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