NAME¶
CGI::Application::Plugin::LinkIntegrity - Make tamper-resisistent links in
CGI::Application
VERSION¶
Version 0.06
SYNOPSIS¶
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 HTML 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
"acct_id" from 73 to 74, the "_checksum" will not match,
and the system will treat it as an intrusion attempt.
Calling link and self_link directly from the template¶
If you use "Template::Toolkit|Template" or
"HTML::Template::Plugin::Dot", you can pass the
"CGI::Application" $self object into the template and call
"link" and "self_link" directly from the template. In your
app:
$template->param(
'app' => $self,
'name' => 'gordon',
'email' => 'gordon@example.com',
);
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 "email" is
the template parameter named email (in this case 'gordon@example.com').
DESCRIPTION¶
"CGI::Application::Plugin::LinkIntegrity" lets you create
tamper-resistent links within your CGI::Application project. When you create
an URL with "link", a "_checksum" is added to the URL:
my $link = $self->link("/account.cgi?rm=balance&acct_id=73");
print $link; # /account.cgi?rm=balance&acct_id=73&_checksum=1d7c4b82d075785de04fa6b98b572691
The checksum is a (cryptographic) hash of the URL, plus a secret string known
only to the server.
If the user attempts to change part of the URL (e.g. a query string parameter,
or the PATH_INFO), then the checksum will not match. The run mode will be
changed to "link_tampered", and the "invalid_checksum"
hook will be called.
You can define the "link_tampered" run mode yourself, or you can use
the default "link_tampered" run mode built into
CGI::Application::Plugin::LinkIntegrity.
You can disable link checking during development by passing a true value to the
"disable" parameter of "$self->link_integrity_config".
METHODS¶
link_integrity_config¶
Configure the CGI::Application::Plugin::LinkIntegrity. Usually, it makes sense
to configure this in the "setup" 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 "disable" in the config file.
The following configuration parameters are available:
- secret
- 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.
- additional_data
- 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.
- checksum_param
- 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 "_checksum".
- link_tampered_run_mode
- The run mode to go to when it has been detected that the user has tampered
with the link. The default is "link_tampered".
See "The link_tampered Run Mode", below.
- digest_module
- Which digest module to use to create the checksum. Typically, this will be
either "Digest::MD5" or "Digest::SHA". However any
module supported by "Digest::HMAC" will work.
The default "digest_module" is "Digest::MD5".
- checksum_generator
- If you want to provide a custom subroutine to make your own checksums, you
can define your own subroutine do it via the "make_checksum"
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;
},
);
- disable
- You can disable link checking entirely by setting "disable" to a
true value. This can be useful when you are developing or debugging the
application and you want the ability to tweak URL params without getting
busted.
link¶
Create a link, and add a checksum to it.
You can add parameters to the link directly in the URL:
my $link = $self->link('/cgi-bin/app.cgi?var=value&var2=value2');
Or you can add them as a hash of parameters after the URL:
my $link = $self->link(
'/cgi-bin/app.cgi',
'param1' => 'value',
'param2' => 'value2',
);
self_link¶
Make a link to the current application, with optional parameters, and add a
checksum to the URL.
my $link = $self->self_link(
'param1' => 'value1',
'param2' => 'value2',
);
"self_link" preserves the value of the current application's
"PATH_INFO". For instance if the current URL is:
/cgi-bin/app.cgi/some/path?foo=bar # PATH_INFO is 'some/path'
Calling:
$self->self_link('bar' => 'baz');
Will produce the URL:
/cgi-bin/app.cgi/some/path?bar=baz
If you want to remove the "PATH_INFO" value or replace it with a new
value, use path_link.
path_link¶
Calling "path_link" is the same as calling "self_link",
except the current value of "PATH_INFO" can be replaced.
my $link = $self->path_link(
'/new/path',
'param1' => 'value1',
'param2' => 'value2',
);
For instance if the current URL is:
/cgi-bin/app.cgi/some/path?foo=bar # PATH_INFO is 'some/path'
Calling:
$self->path_link('/new/path');
Will produce the URL:
/cgi-bin/app.cgi/new/path?foo=bar
If you want to remove "PATH_INFO" 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 "PATH_INFO" that was passed to the
current application, use self_link instead.
RUN MODES¶
The link_tampered Run Mode¶
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 "link_tampered_runmode" param to
"link_integrity_config".
CGI::Application::Plugin::LinkIntegrity provides a default
"link_tampered" 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;
}
HOOKS¶
When a link is followed that doesn't match the checksum, the
"invalid_checksum" 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;
}
AUTHOR¶
Michael Graham, "<mag-perl@occamstoothbrush.com>"
ACKNOWLEDGEMENTS¶
This module was based on the checksum feature originally built into Richard
Dice's CGI::Application::Framework.
BUGS¶
Please report any bugs or feature requests to
"bug-cgi-application-plugin-linkintegrity@rt.cpan.org", 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 & LICENSE¶
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.