Exploiting Stored XSS: Bypassing HttpOnly Cookies and Chaining Vulnerabilities

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Introduction

Stored Cross-Site Scripting (XSS) remains a potent threat, even when security measures like HttpOnly, Secure, and `SameSite=Strict` cookies are implemented. This article explores how attackers can leverage XSS to bypass these protections and escalate privileges by chaining vulnerabilities like exposed API documentation.

Learning Objectives

  • Understand how XSS can bypass `HttpOnly` cookies using `fetch` with credentials: 'include'.
  • Learn how information disclosure (e.g., Swagger docs) aids attackers in crafting precise API exploits.
  • Discover mitigation strategies to prevent XSS and API abuse.

1. Bypassing HttpOnly Cookies with XSS

Command/Code Snippet:

fetch('/api/user/teammate', { 
method: 'POST', 
credentials: 'include', 
headers: { 'Content-Type': 'application/json' }, 
body: JSON.stringify({ email: '[email protected]', role: 'admin' }) 
});

Step-by-Step Guide:

  1. Identify XSS Entry Point: Inject a malicious script into a vulnerable input field (e.g., comment section).
  2. Craft Payload: Use `fetch` with `credentials: ‘include’` to force the browser to attach session cookies.
  3. Target API Endpoints: Send authenticated requests to create/administer accounts or exfiltrate data.

Why It Works: `HttpOnly` prevents JavaScript from reading cookies but doesn’t block them from being sent with requests.

2. Exploiting Exposed Swagger Documentation

Command/Code Snippet:

curl -X GET https://victim.com/api-docs/swagger.json | jq '.paths'

Step-by-Step Guide:

  1. Discover Swagger Docs: Look for `/swagger.json` or `/openapi.json` on the target site.
  2. Enumerate Endpoints: Extract API routes and parameters to craft precise attacks.
  3. Chain with XSS: Use the XSS payload to call these endpoints with stolen cookies.

Mitigation: Restrict access to API documentation and enforce authentication.

3. Preventing XSS Attacks

Code Snippet (CSP Header):

Content-Security-Policy: default-src 'self'; script-src 'unsafe-inline' 'unsafe-eval'

Steps to Implement:

  1. Use CSP: Restrict inline scripts and external sources.
  2. Input Sanitization: Escape user inputs with libraries like DOMPurify.
  3. Cookie Flags: Ensure `SameSite=Strict` and `Secure` are set.

4. Hardening APIs Against Abuse

Code Snippet (Rate Limiting):

limit_req_zone $binary_remote_addr zone=api_limit:10m rate=100r/m;

Steps:

  1. Rate Limiting: Throttle requests to prevent brute force.
  2. Auth Validation: Require CSRF tokens for state-changing requests.
  3. Logging: Monitor unusual API activity (e.g., rapid account creation).

5. Detecting XSS Payloads

Command (Log Analysis):

grep -E "<script|alert(" /var/log/nginx/access.log

Steps:

  1. Log Inspection: Search for script patterns in HTTP logs.
  2. WAF Rules: Deploy regex-based filters to block XSS attempts.

What Undercode Say

  • Key Takeaway 1: `HttpOnly` cookies aren’t foolproof—XSS can still abuse them to perform authenticated actions.
  • Key Takeaway 2: Information disclosure (e.g., Swagger docs) turns minor flaws into critical risks when chained.

Analysis:

Roie Marciano’s demo highlights how “defense in depth” fails if one layer is weak. The combo of XSS and exposed API docs allowed privilege escalation without stealing cookies. Future attacks will increasingly exploit such chains, emphasizing the need for:
– Strict API documentation access controls.
– Client-side protections like CSP.
– Proactive vulnerability scanning for info leaks.

Prediction:

As APIs dominate modern apps, attackers will pivot to automating XSS-to-API exploits, making Swagger/doc hardening as critical as input sanitization.

For more cybersecurity insights, follow Roie Marciano on LinkedIn.

IT/Security Reporter URL:

Reported By: Roiemarciano Stored – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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