Web App Penetration Testing Guide Basics

How to Penetration Test a Web Application (Full Beginner-Friendly Guide)

Understanding Web Application Penetration Testing​

Web Application Penetration Testing - often called Web App Pentesting - is the process of legally and ethically attacking a web application to discover security weaknesses before a real hacker does . It’s one of the most important skills in cybersecurity today, especially as more businesses move services online.

A pentester behaves like a real attacker, but with permission. They analyze the app, test its defenses, and find vulnerabilities such as SQL Injection, XSS, CSRF, Unauthorized Access, and much more. The main goal is to help developers fix problems before they cause damage.

If you're learning cybersecurity, working in bug bounty hunting, or securing your own web application, this guide will help you understand the “How To PenTest” workflow clearly and practically.

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Why Web App Pentesting Matters​

Web applications store sensitive data: login information, personal details, payments, business systems, and more. A single vulnerability can expose everything .
That’s why penetration testing is essential - it helps:

• Protect sensitive data
• Stop real-world hackers
• Strengthen application security
• Discover weak authentication and session handling
• Find misconfigurations before attackers do
• Improve bug bounty success

In short: better security = safer users + safer business.

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The Pentesting Workflow: Step by Step​

A professional pentest follows a structured methodology. Whether you're using GitHub projects like How_To_PenTest, OWASP guidelines, or your own notes, you should always follow these steps:

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Reconnaissance (Information Gathering) ​

This is the first and most important step. You collect everything possible about your target without attacking it yet. This helps you understand how the app works.

Tools for Recon:​

• Nmap - scan ports & services
• WhatWeb - identify technologies
• Whois - gather domain information
• Sublist3r - find subdomains
• Dirsearch / Gobuster - brute-force hidden directories

Example: Finding Subdomains​

sublist3r -d targetsite.com

Example: Tech Stack Fingerprinting​

whatweb https://targetsite.com

This phase is like turning on the lights before entering the room - you need visibility.

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Scanning & Enumerating the Application ​

Now that you know what's running, it's time to scan it for weaknesses.

Common Tools:​

• OWASP ZAP
• Burp Suite Scanner
• Nikto
• SSL Labs Testing

Example: Basic Vulnerability Scanning​

nikto -h https://targetsite.com

This phase helps find outdated systems, weak headers, insecure cookies, default pages, and misconfigurations.

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Manual Vulnerability Testing ​

Automated tools are great… but manual testing is where real pentesting happens. Here you test the application by hand using logic, creativity, and hacker mindset.

Below are the most common vulnerabilities:

Testing for SQL Injection ​

SQL Injection happens when an application fails to sanitize inputs, allowing attackers to execute SQL queries.

Common Test Payload​

'
"
' OR '1'='1
" OR "1"="1

Example Attack URL:​

https://target.com/product?id=5'

If the page breaks or returns unexpected data, this means potential SQL Injection.

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Testing for Cross-Site Scripting (XSS) ​

XSS happens when the application displays unvalidated user input, allowing attackers to run JavaScript.

Common XSS Test Payload:​

<script>alert('XSS')</script>

Example injection field:​

• Search bar
• Username field
• Comments
• URL parameters

If a pop-up appears or code executes → vulnerability confirmed.

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Testing for Broken Authentication ​

Authentication issues are among the most dangerous.

What to test:​

• Weak password policy
• Brute-force protection
• Session expiration
• Token predictability
• “Remember me” cookies

Example: Checking for Weak Session IDs​

If session IDs look like this: 12345
Instead of: KHG73JHD82SA900SSJ3SHD872
Then the system is weak and predictable.

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Testing File Upload Vulnerabilities ​

Web apps that allow file uploads are very risky - hackers may try to upload scripts.

Example PHP Shell disguised as an image:​

<?php system($_GET['cmd']); ?>

Named as: image.jpg.php

Upload Bypass Techniques:​

• Double extensions
• MIME-type spoofing
• Missing file validations

A secure app should check:

• File type
• File size
• Content
• Destination path

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Testing Access Control (Authorization) ​

Authorization problems allow normal users to behave like administrators.

Example: IDOR (Insecure Direct Object Reference)​

Try changing user ID in URL:

https://targetsite.com/profile?id=1001

→ you see another user's profile?
That’s a critical vulnerability.

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Business Logic Testing ​

This is one of the hardest parts because it's not about coding - it's about logic.

Examples:​

• Skipping steps in checkout
• Reusing coupons multiple times
• Accessing paid content for free
• Changing prices via hidden fields

Apps often break because developers don’t expect users to behave maliciously.

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Code Sample: Simple HTTP Request Test (Python) ​

This script fetches all visible links from the homepage:

import requests
from bs4 import BeautifulSoup

url = "https://targetsite.com"
response = requests.get(url)

soup = BeautifulSoup(response.text, "html.parser")
links = {a['href'] for a in soup.find_all("a", href=True)}

for link in links:
    print(link)

Useful during recon and early enumeration.

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Reporting - The Most Important Step ​

A pentest report should include:

• Vulnerability name
• Description
• Impact
• Severity level
• Proof of Concept
• Steps to reproduce
• Recommended fix

A clean, professional report makes you look like a real security engineer - not just someone running tools.

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Tools Commonly Recommended in GitHub Projects Like How_To_PenTest​

• Burp Suite
• OWASP ZAP
• Nmap
• Gobuster / Dirsearch
• SQLMap for automated SQL Injection discovery
• WFuzz for fuzzing endpoints
• Postman / cURL for API testing

These tools give you everything you need for professional-grade pentests.

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Ethical Rules You Must Follow ​

Before testing a website:

• You must have legal permission
• You must respect the scope
• Never damage data
• Never download sensitive information
• Never perform Denial-of-Service attacks unless allowed
• Always report findings responsibly

Pentesting without permission = illegal hacking.

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https://github.com/Fckroun/How_To_PenTest
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Final Thoughts ​

Learning how to penetration test a web application is one of the best investments you can make in cybersecurity. Whether you're following GitHub guides like How_To_PenTest, studying OWASP, or exploring real-life bug bounty programs, the process is the same:

1. Recon
2. Scan
3. Manual testing
4. Exploitation
5. Reporting

Keep practicing, stay ethical, and challenge yourself with new applications. The more you test, the more skilled you become .