Unveiling a CORS Misconfiguration: A Journey to Account Takeover

Introduction

Welcome to my first deep dive into a recent discovery that highlights the critical importance of proper web security configurations. In this article, I’ll walk you through how a misconfigured Cross-Origin Resource Sharing (CORS) policy led to a full account takeover (ATO) and bypassed two-factor authentication (2FA). Whether you’re a seasoned developer or just starting out, understanding these vulnerabilities can help you safeguard your applications.

The Discovery

While exploring a popular service, I stumbled upon a CORS misconfiguration that allowed unauthorized access to sensitive data. This service, designed to protect user privacy while enabling legitimate interactions, inadvertently exposed itself to potential exploitation.

Methodology

1. Initial Setup:

  • Account Creation: I began by creating a new account on the target’s signup page, opting for the «Pro» version with a free trial.
  • 2FA Activation: After registration, I enabled two-factor authentication and added a security key for password-less logins.

2. Traffic Analysis:

  • Tool Used: Burp Suite was employed to monitor traffic and analyze requests.
  • Key Observation: A GET request revealed sensitive information, including a session token in the response header.

Understanding CORS Misconfigurations

What is CORS?
CORS is a security feature implemented by web browsers to control resource sharing across different origins, complementing the Same-Origin Policy (SOP).

Common Misconfigurations:

  • Allowing wildcard origins (*) or improperly configured Access-Control-Allow-Origin headers can expose sensitive data.

Exploiting the Misconfiguration

1. Initial Attempts:

  • Origin Header Manipulation: I experimented with various origin headers, but unauthorized sources were consistently denied.

2. Breakthrough:

  • Trusted Subdomain: By appending any string to a trusted subdomain, requests were accepted, revealing a critical flaw.

3. Proof of Concept (PoC):

  • Script Execution: A malicious script was crafted to exploit the vulnerability, successfully reading sensitive responses from an arbitrary origin.

Account Takeover

1. Session Token Exposure:

  • Despite being marked HttpOnly, the session token was exposed in the response, allowing unauthorized access.

2. Bypassing 2FA:

  • By injecting the session token into a new browser session, I gained full control over the account, bypassing all authentication measures.

Conclusion

This exploration underscores the importance of thoroughly examining CORS configurations and understanding their potential impact on security. Always scrutinize dynamic origin reflections in responses, as they may reveal deeper vulnerabilities.

Tips for Developers:

  • Regularly audit your CORS policies to ensure they are not overly permissive.
  • Consider implementing additional security measures, such as Content Security Policy (CSP), to mitigate risks.

By sharing this experience, I hope to raise awareness about the nuances of web security and encourage proactive measures to protect user data. Stay vigilant and keep learning!

Timeline

  • Report Submission: February 19, 2025
  • Initial Response: February 19, 2025
  • Partial Fix: February 19, 2025
  • Complete Fix: March 9, 2025
  • Reward Issued: March 14, 2025

Thank you for joining me on this journey into the world of cybersecurity. Feel free to share your thoughts or questions in the comments below!