CGI pentesting

👉 Overview

👀 What ?

CGI or Common Gateway Interface pentesting is a method of assessing the security of web servers and web applications by exploiting vulnerabilities in CGI scripts. These scripts, often written in languages like Perl, Python or Bash, are used to interface between a web server and applications. The fundamental concepts of CGI pentesting involve understanding the functioning of CGI scripts, identifying potential security weaknesses, and exploiting these vulnerabilities to gain unauthorized access or disrupt services.

🧐 Why ?

Understanding CGI pentesting is crucial as CGI scripts are commonly used in web development. However, they can often be the weakest link in a web application's security. Poorly written or outdated CGI scripts can allow an attacker to execute arbitrary commands, enabling them to steal sensitive data, deface websites, or even take control of the server. Therefore, it's essential for anyone involved in cybersecurity, particularly penetration testers, to understand how to identify and exploit vulnerabilities in CGI scripts.

⛏️ How ?

CGI Pentesting typically involves steps like reconnaissance (gathering information about the target system), scanning (using tools like Nmap or Nessus to identify running services and open ports), enumeration (identifying the versions of the CGI scripts and looking for known vulnerabilities), exploitation (attempting to exploit the identified vulnerabilities), and post-exploitation (maintaining access and cleaning up). It's essential to follow ethical guidelines during the process, only performing pentesting on systems where you have permission.

⏳ When ?

The practice of CGI pentesting started becoming more prevalent with the growth of the internet and the need for improved cybersecurity. Its usage has become even more critical with the rise of web-based applications and services in the past decade.

⚙️ Technical Explanations

CGI, or Common Gateway Interface, scripts serve a crucial role in web server operations. They function as the go-between for the server and any other software running on it. Whenever a user sends a request that needs processing beyond just retrieving a static web page, a CGI script handles it.

The script first processes the incoming request. This could involve various operations based on the specific nature of the request, such as extracting form data, executing database queries, or performing computational tasks. The script then communicates with the necessary software on the server to execute these operations. For instance, it might interact with a database management system to fetch or update data, or with other server-side scripts to perform more complex operations.

Once the script has executed the necessary functions, it then prepares the response in the form of an HTML document. This document is then served back to the user's browser, allowing the requested operations to be completed, and the results displayed to the user.

From a security perspective, CGI scripts pose potential risks if not properly managed. Specifically, if an attacker can manipulate the data being sent to the CGI script, they can cause the script to behave unexpectedly. For example, if user input from a form on a web page is not correctly sanitized before being processed by the CGI script, an attacker could inject malicious commands. This could lead to serious security breaches, including the execution of arbitrary commands on the server, unauthorized access to sensitive data, or even total control over the server.

Thus, understanding and properly managing CGI scripts and their vulnerabilities is a critical aspect of web security. This includes ensuring proper input validation and sanitization, keeping scripts updated to the latest versions, and regularly conducting penetration testing to identify and address potential security weaknesses.

Let's assume we have a simple CGI script written in Perl that takes user input from a form and uses it to query a database. Here's a simplified version of such a script:

#!/usr/bin/perl
use CGI;
use DBI;

my $q = new CGI;
my $user_input = $q->param('user_input');

my $dbh = DBI->connect("DBI:mysql:database:localhost", "username", "password");
my $sth = $dbh->prepare("SELECT * FROM users WHERE name = '$user_input'");
$sth->execute();

while (my @row = $sth->fetchrow_array) {
    print "@row\\n";
}
$dbh->disconnect;

This script does the following:

1. It takes user input from a form via the CGI module.
2. It connects to a MySQL database using the DBI module.
3. It prepares a SQL query, inserting the user input directly into the query.
4. It executes the query and prints the results.

The security issue here lies in step 3, where the script inserts the user input directly into the SQL query. This can lead to a common vulnerability known as SQL injection. For instance, if a user enters a value like ' OR '1'='1, the SQL query becomes SELECT * FROM users WHERE name = '' OR '1'='1', which will return all users.

To mitigate this, we should sanitize the user input before using it in the SQL query. We could do this using the quote method provided by the DBI module:

$user_input = $dbh->quote($user_input);
my $sth = $dbh->prepare("SELECT * FROM users WHERE name = $user_input");

This will ensure that any special characters in the user input are properly escaped, preventing them from altering the SQL query. Regularly conducting penetration tests can help identify such vulnerabilities and ensure the security of your CGI scripts.