Michael Graham

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.

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 paramter 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

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.

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::SHA1. 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.

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',
    );

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.

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

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.




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