cgi security

CGI Security

COEN 351

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.

For example, a pretty-printer for ASCII art.

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

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 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 tjschwarz@scu.edu < /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 tjschwarz@scu.edu < /etc/passwd

CGI Security

Interpretation Example Clearly, we need to be careful about

filtering out bad input. Other examples include

SQL injection attacks Access to resources

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

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

my $App = /temp/app.exe’;my $q = new CGI;my $string = $q->param( “string” );unless ( $string ) { error( $q, “Need string parameter”);}if ($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;

This excludes characters $ \ “ ` ; &

CGI Security Interpretation Example

We want to only allow strings that are alpha-numerical, have underscores, hyphens, periods, question marks, and exclamation points.

However, the strategy of enumerating bad characters needs to be amended to exclude all possible escape sequences:

ASCII / Unicode escapes Foreign language symbols Double escapes

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( $q, “Need string parameter”);}if ($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;

This lists good characters:

alpha-numeric

. ! ? -

CGI Security

Interpretation Example This is much better.

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

What about users using a different character set?

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 the child process call

exec. This will prevent part of malicious

user input to end up as a command.

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;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;

This script bypasses the shell.

This call to “open” tells Perl to fork and create a child process with a pipe connected to it.

The child process is a copy of the current executing script and continues from the same point.

Parent receives $pid of child process.

Child receives $pid of zero.

Child process calls exec, which calls the app on the input.

Parent maintains pipe to the app.

CGI Security

DO NOT TRUST INPUT Data in hidden fields can be changed

by the user. Referer data can be changed. Data in cookies can be changed.

CGI Security

Hidden Forms are not secure:

CGI Security

Hidden forms are not secure: This script generates a new URL https://localhost/cgi/buy.cgi?price=30.00&name=Super+Blaster+3000&quantity=1&submit=Order.

User can simply edit this URL and get another price posted to the webserver.

CGI Security

Hidden forms are not secure Therefore, we use the Post-method.

However: Attacker can save the webpage. Edit the form-field Change price that way.

CGI script cannot distinguish which webpage called.

CGI Security Other possibility:

Trust the referer field in the header.

Someone using a standard browser cannot alter easily the referer field.

However, you can send HTTP commands directly with netcat, …

my $server = quotemeta( $ENV{HTTP_HOST} || $ENV(SERVER_NAME) );

unless ($ENV{HTTP_REFERER} =~ m|^https?://$server/| ) {

error( $q, “Invalid referring URL.” );}

CGI Security

Do not trust unencoded cookies. User can access and alter the cookie

with any number of tools.

CGI Security

Countermeasures: Protect data with encryption.

Use SSL to protect data integrity and content in transit.

Validate any information that the user can change by signature or digest.

CGI Security

Protection Mechanism against alteration Use a secure digest:

Concatenate values in hidden form with a secret value.

Store the hash of the resulting string. When you receive data, verify the hash.

CGI Security

Protection Mechanism against alteration

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.