Pablo Guides

# Colly: The Ultimate Guide to Web Scraping in PenTesting

## Section 1: Introduction & Installation

### Introduction to Colly

Colly is a powerful, fast, and elegant web scraping framework for the Go programming language. As web applications become increasingly complex, penetration testers need precise tools that can efficiently gather data and analyze web applications for vulnerabilities. Colly serves that need by allowing testers to automate the data extraction process while adhering to ethical scraping practices.

Web scraping is crucial in penetration testing because it enables security professionals to gather intelligence about a target application. This intelligence can lead to identifying vulnerabilities, mapping application structures, and gathering data that may be exploited during an assessment.

### Installing Colly on Kali Linux

To get started with Colly, you will need to have a working installation of Go on your Kali Linux system. Here’s how to install and configure Colly:

#### Step 1: Install Go

1. Open your terminal on Kali Linux.
2. Update your package manager:

   sudo apt update

 

   sudo apt install golang

 

   go version

 

#### Step 2: Set Up Go Environment Variables

1. Open your `.bashrc` or `.zshrc` file:

   nano ~/.bashrc

 

   export GOPATH=$HOME/go

   export PATH=$PATH:$GOPATH/bin

 

   source ~/.bashrc

 

#### Step 3: Install Colly

With Go installed, you can now install the Colly library.

1. Create a new directory for your Go projects:

   mkdir -p ~/go/src/my-scraper

   cd ~/go/src/my-scraper

 

   go mod init my-scraper

 

   go get -u github.com/gocolly/colly/v2

 

Now you have Colly installed and ready to use for web scraping.

### Configuration

To configure Colly for optimal performance in penetration testing, you'll typically want to set up a user agent, handle delays between requests, and manage error handling. Here’s how to do that:

"`go
package main

import (
"github.com/gocolly/colly/v2"
"time"
)

func main() {
// Create a new collector
c := colly.NewCollector()

// Set user agent
c.UserAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.3"

// Set request delay
c.Limit(&colly.Limit{
Delay: 2 * time.Second,
})

// Set error handler
c.OnError(func(r *colly.Response, err error) {
// Log error details
log.Printf("Request failed with response %v and error %vn", r, err)
})

// Your scraping logic goes here

}
"`

### Step-by-Step Usage of Colly

#### Basic Example of a Scraper

Let’s start by building a simple web scraper that collects the titles of articles from a blog. The following example demonstrates how to extract data from a sample blog page.

"`go
package main

import (
"fmt"
"github.com/gocolly/colly/v2"
)

func main() {
// Create a new collector
c := colly.NewCollector()

// On every HTML element which has a class attribute "post-title"
c.OnHTML(".post-title", func(e *colly.HTMLElement) {
fmt.Println("Article Title: ", e.Text)
})

// Start the scraping process
c.Visit("https://example-blog.com")
}
"`

### Real-World Use Cases

#### 1. Data Gathering for Vulnerability Assessment

In penetration testing, gathering data about a target web application is crucial. You can use Colly to enumerate endpoints, resources, and parameters that may be vulnerable. Consider this example where we scrape for URLs:

"`go
package main

import (
"fmt"
"github.com/gocolly/colly/v2"
)

func main() {
c := colly.NewCollector()

c.OnHTML("a[href]", func(e *colly.HTMLElement) {
fmt.Println("Found link:", e.Attr("href"))
})

c.Visit("https://target-website.com")
}
"`

#### 2. Scraping for Sensitive Information

Colly can also be useful in extracting potentially sensitive information exposed on public web pages. For example, if you’re auditing a blog that lists employee names and emails:

"`go
package main

import (
"fmt"
"github.com/gocolly/colly/v2"
)

func main() {
c := colly.NewCollector()

c.OnHTML(".employee-info", func(e *colly.HTMLElement) {
name := e.ChildText(".name")
email := e.ChildText(".email")
fmt.Printf("Employee: %s, Email: %sn", name, email)
})

c.Visit("https://example-company.com/employees")
}
"`

### Detailed Technical Explanations

Colly relies on Go's concurrency features, making it extremely fast and capable of handling multiple requests simultaneously. When scraping web applications, here are some important concepts you should understand:

– **Selectors**: Colly uses CSS selectors to find elements on the page. Familiarize yourself with CSS syntax for more effective scraping.

– **Callbacks**: Use callbacks to handle events like visiting a page, finding elements, or encountering errors. This allows for granular control over the scraping process.

– **Throttling and Rate Limiting**: Implementing delays between requests is critical to avoid overwhelming the target server and to stay ethical in your data collection. The `colly.Limit` function is useful for this purpose.

– **Error Handling**: Always include error handling to catch potential issues, such as network errors or unexpected HTML structure changes.

### External Reference Links

– [Colly Documentation](https://pkg.go.dev/github.com/gocolly/colly/v2)
– [Go Programming Language](https://golang.org/doc/)
– [Ethical Web Scraping Practices](https://www.scrapingbee.com/blog/ethical-web-scraping/)
– [Web Application Vulnerability Scanning](https://owasp.org/www-project-web-security-testing-guide/latest/)

With the installation and basic usage covered, you can now start using Colly in your penetration testing toolkit. In the following sections, we will dive deeper into advanced scraping techniques, handling JavaScript-rendered pages, and dealing with CAPTCHAs.

—

Made by pablo rotem / פבלו רותם