Make tamper-resisistent links in cgi::application
Version 0.06
In your application:
use base 'CGI::Application';
use CGI::Application::Plugin::LinkIntegrity;
sub setup {
my $self = shift;
$self->link_integrity_config(
secret => 'some secret string known only to you and me',
);
}
sub account_info {
my $self = shift;
my $account_id = get_user_account_id();
my $template = $self->load_tmpl('account.html');
$template->param(
'balance' => $self->link("/account.cgi?rm=balance&acct_id=$account_id");
'transfer' => $self->link("/account.cgi?rm=transfer&acct_id=$account_id");
'withdrawal' => $self->link("/account.cgi?rm=withdrawl&acct_id=$account_id");
);
}
In your template:
<h1>Welcome to The Faceless Banking Corp.</h1> <h3>Actions:</h3> <br /><a href="<TMPL_VAR NAME="balance">">Show Balance</a> <br /><a href="<TMPL_VAR NAME="transfer">">Make a Transfer</a> <br /><a href="<TMPL_VAR NAME="withdrawal">">Get Cash</a>
This will send the following \s-1HTML\s0 to the browser:
<h1>Welcome to The Faceless Banking Corp.</h1> <h3>Actions:</h3> <br /><a href="/account.cgi?rm=balance&acct_id=73&_checksum=1d7c4b82d075785de04fa6b98b572691">Show Balance</a> <br /><a href="/account.cgi?rm=transfer&acct_id=73&_checksum=d41d8cd98f00b204e9800998ecf8427e">Make a Transfer</a> <br /><a href="/account.cgi?rm=withdrawl&acct_id=73&_checksum=3c5ad17bdeef3c4281abd39c6386cfd6">Get Cash</a>
The URLs created are now tamper-resistent. If the user changes \*(C`acct_id\*(C' from 73 to 74, the \*(C`_checksum\*(C' will not match, and the system will treat it as an intrusion attempt.
If you use \*(C`Template::Toolkit|Template\*(C' or \*(C`HTML::Template::Plugin::Dot\*(C', you can pass the \*(C`CGI::Application\*(C' $self object into the template and call \*(C`link\*(C' and \*(C`self_link\*(C' directly from the template. In your app:
$template->param( 'app' => $self, 'name' => 'gordon', 'email' => '[email protected]', );
And in your template you can use
# Template::Toolkit syntax <a href="[% app.self_link('name', name, 'email', email %]">...</a>
# HTML::Template::Plugin::Dot syntax <a href="<TMPL_VAR NAME="app.self_link('name', name, 'email', email">">...</a>
# Petal syntax <a href="http://www.example.com" tal:attributes="href app/self_link('name', name, 'email', email)">...</a>
Note that in the parameters of the call to << link >>, items enclosed in quotes are treated as literal parameters and barewords are treated as template params. So 'email' is the literal string, and \*(C`email\*(C' is the template parameter named email (in this case '[email protected]').
\*(C`CGI::Application::Plugin::LinkIntegrity\*(C' lets you create tamper-resistent links within your CGI::Application project. When you create an \s-1URL\s0 with \*(C`link\*(C', a \*(C`_checksum\*(C' is added to the \s-1URL:\s0
my $link = $self->link("/account.cgi?rm=balance&acct_id=73"); print $link; # /account.cgi?rm=balance&acct_id=73&_checksum=1d7c4b82d075785de04fa6b98b572691
The checksum is a (cryptographic) hash of the \s-1URL\s0, plus a secret string known only to the server.
If the user attempts to change part of the \s-1URL\s0 (e.g. a query string parameter, or the \s-1PATH_INFO\s0), then the checksum will not match. The run mode will be changed to \*(C`link_tampered\*(C', and the \*(C`invalid_checksum\*(C' hook will be called.
You can define the \*(C`link_tampered\*(C' run mode yourself, or you can use the default \*(C`link_tampered\*(C' run mode built into CGI::Application::Plugin::LinkIntegrity.
You can disable link checking during development by passing a true value to the \*(C`disable\*(C' parameter of \*(C`$self->link_integrity_config\*(C'.
Configure the CGI::Application::Plugin::LinkIntegrity. Usually, it makes sense to configure this in the \*(C`setup\*(C' method of your application's base class:
use CGI::Application::Plugin::LinkIntegrity; use base 'CGI::Application'; package My::Project;
sub setup { my $self = shift;
$self->run_modes(['bad_user_no_biscuit']); $self->link_integrity_config( secret => 'some secret string known only to you and me', link_tampered_run_mode => 'bad_user_no_biscuit', digest_module => 'Digest::MD5', disable => 1, ); }
Or you can pull in this configuration info from a config file. For instance, with using CGI::Application::Config::Context:
use CGI::Application::Plugin::LinkIntegrity; use CGI::Application::Plugin::Config::Context;
use base 'CGI::Application'; package My::Project;
sub setup { my $self = shift;
$self->conf->init( file => 'app.conf', driver => 'ConfigGeneral', );
my $config = $self->conf->context;
$self->link_integrity_config( $config->{'LinkIntegrity'}, additional_data => sub { my $self = shift; return $self->session->id; }, );
my $link_tampered_rm = $config->{'LinkIntegrity'}{'link_tampered_run_mode'} || 'link_tampered';
$self->run_modes([$link_tampered_rm]); }
Then in your configuration file:
<LinkIntegrity> secret = some REALLY secret string link_tampered_run_mode = bad_user_no_biscuit hash_algorithm = SHA1 disable = 1 </LinkIntegrity>
This strategy allows you to enable and disable link checking on the fly by changing the value of \*(C`disable\*(C' in the config file.
The following configuration parameters are available:
A string known only to your application. At a commandline, you can generate a secret string with md5: $ perl -MDigest::MD5 -le"print Digest::MD5::md5_hex($$, time, rand(42));" Or you can use Data::UUID: $ perl -MData::UUID -le"$ug = new Data::UUID; $uuid = $ug->create; print $ug->to_string($uuid)" If someone knows your secret string, then they can generate their own checksums on arbitrary data that will always pass the integrity check in your application. That's a Bad Thing, so don't let other people know your secret string, and don't use the default secret string if you can help it.
You can pass constant additional data to the checksum generator for every link. $self->link_integrity_config( secret => 'really secret', additional_data => 'some other secret data', } For instance, to stop one user from following a second user's link, you can add a user-specific component to the session, such as the user's session id: $self->link_integrity_config( secret => 'really secret', additional_data => sub { my $self = shift; return $self->session->id; } } You can pass a string instead of a subroutine. But in the case of the user's session, a subroutine is useful so that you get the value of the user's session at the time when the checksum is generated, not at the time when the link integrity system is configured.
The name of the checksum parameter, which is added to the query string and contains the cryptographic checksum of link. By default, this parameter is named \*(C`_checksum\*(C'.
The run mode to go to when it has been detected that the user has tampered with the link. The default is \*(C`link_tampered\*(C'. See \*(L"The link_tampered Run Mode\*(R", below.
Which digest module to use to create the checksum. Typically, this will be either \*(C`Digest::MD5\*(C' or \*(C`Digest::SHA\*(C'. However any module supported by \*(C`Digest::HMAC\*(C' will work. The default \*(C`digest_module\*(C' is \*(C`Digest::MD5\*(C'.
If you want to provide a custom subroutine to make your own checksums, you can define your own subroutine do it via the \*(C`make_checksum\*(C' param. Here's an example of one that uses Digest::SHA2: $self->link_integrity_config( checksum_generator => sub { my ($url, $secret) = @_; require Digest::SHA2;
my $ctx = Digest::SHA2->new(); $ctx->add($url . $secret);
return $ctx->hexdigest; }, );
You can disable link checking entirely by setting \*(C`disable\*(C' to a true value. This can be useful when you are developing or debugging the application and you want the ability to tweak \s-1URL\s0 params without getting busted.
Create a link, and add a checksum to it.
You can add parameters to the link directly in the \s-1URL:\s0
my $link = $self->link('/cgi-bin/app.cgi?var=value&var2=value2');
Or you can add them as a hash of parameters after the \s-1URL:\s0
my $link = $self->link( '/cgi-bin/app.cgi', 'param1' => 'value', 'param2' => 'value2', );
Make a link to the current application, with optional parameters, and add a checksum to the \s-1URL\s0.
my $link = $self->self_link( 'param1' => 'value1', 'param2' => 'value2', );
\*(C`self_link\*(C' preserves the value of the current application's \*(C`PATH_INFO\*(C'. For instance if the current \s-1URL\s0 is:
/cgi-bin/app.cgi/some/path?foo=bar # PATH_INFO is 'some/path'
Calling:
$self->self_link('bar' => 'baz');
Will produce the \s-1URL:\s0
/cgi-bin/app.cgi/some/path?bar=baz
If you want to remove the \*(C`PATH_INFO\*(C' value or replace it with a new value, use path_link.
Calling \*(C`path_link\*(C' is the same as calling \*(C`self_link\*(C', except the current value of \*(C`PATH_INFO\*(C' can be replaced.
my $link = $self->path_link( '/new/path', 'param1' => 'value1', 'param2' => 'value2', );
For instance if the current \s-1URL\s0 is:
/cgi-bin/app.cgi/some/path?foo=bar # PATH_INFO is 'some/path'
Calling:
$self->path_link('/new/path');
Will produce the \s-1URL:\s0
/cgi-bin/app.cgi/new/path?foo=bar
If you want to remove \*(C`PATH_INFO\*(C' entirely, call one of the following:
$self->path_link; $self->path_link(undef, 'param1' => 'val1', 'param2 => 'val2' ...); $self->path_link('', 'param1' => 'val1', 'param2 => 'val2' ...);
If you want to keep the existing \*(C`PATH_INFO\*(C' that was passed to the current application, use self_link instead.
Your application is redirected to this run mode when it has been detected that the user has tampered with the link. You can change the name of this run mode by changing the value of the \*(C`link_tampered_runmode\*(C' param to \*(C`link_integrity_config\*(C'.
CGI::Application::Plugin::LinkIntegrity provides a default \*(C`link_tampered\*(C' run mode, which just displays a page with some stern warning text.
You can define your own as follows:
sub link_tampered { my $self = shift; my $template = $self->load_template('stern_talking_to'); return $template->output; }
When a link is followed that doesn't match the checksum, the \*(C`invalid_checksum\*(C' hook is called. You can add a callback to this hook to do some cleanup such as deleting the user's session. For instance:
sub setup { my $self = shift; $self->add_callback('invalid_checksum' => \&bad_user); }
sub bad_user { my $self = shift;
# The user has been messing with the URLs, possibly trying to # break into the system. We don't tolerate this behaviour. # So we delete the user's session:
$self->session->delete; }
Michael Graham, \*(C`<[email protected]>\*(C'
This module was based on the checksum feature originally built into Richard Dice's CGI::Application::Framework.
Please report any bugs or feature requests to \*(C`[email protected]\*(C', or through the web interface at <http://rt.cpan.org>. I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
Copyright 2005 Michael Graham, All Rights Reserved.
This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.