Stored XSS Vulnerability Exploiting GitHub Authentication on WebComponents.org

Cross-site scripting (XSS) vulnerabilities remain a significant threat to web application security, often enabling attackers to hijack user credentials or perform unauthorized actions. A notable example is the stored XSS vulnerability discovered in the popular WebComponents.org platform, which allowed attackers to act on behalf of authenticated GitHub users by exploiting their authentication tokens.

Introduction to the WebComponents.org Vulnerability

WebComponents.org is a platform where developers can publish and share Polymer elements and other web components. In August 2018, a security vulnerability was identified where an attacker could inject malicious JavaScript code by misusing the element’s homepage URL field. This flaw enabled Stored XSS attacks, compromising users who had previously authenticated with GitHub on the site.

Understanding Stored XSS and OAuth Authentication Risks

Stored XSS occurs when malicious scripts are permanently saved in the target application’s data and served to other users. Unlike reflected XSS, stored XSS poses higher risks due to persistent infection.

The vulnerability specifically exploited GitHub OAuth authentication integration. If a user had authenticated on WebComponents.org with their GitHub account, attackers could extract the GitHub authorization code embedded in redirected URLs, enabling unauthorized API actions on behalf of the user.

How the Attack Worked: Step-by-Step

1. Publication of Malicious Polymer Element: An attacker published a Polymer element to GitHub and set its repository homepage URL to a javascript: URL containing malicious code (e.g., javascript:alert(document.domain)).
2. Publishing via WebComponents.org: They then published this element through WebComponents.org’s publishing tool.
3. Execution of Stored XSS: When users visited the element page on WebComponents.org and clicked the homepage link, the injected JavaScript ran in the context of WebComponents.org.
4. Hijacking GitHub OAuth Code: The malicious script created a hidden iframe loading the GitHub OAuth authorization URL pointing back to WebComponents.org. Since the iframe was same-origin, the attacker’s script accessed the redirected URL containing the GitHub code parameter.
5. Automated Unauthorized Actions: Using the stolen OAuth code, the attacker sent a POST request to WebComponents.org’s API to star any public GitHub repository on behalf of the authenticated user.

Technical Details and Exploit Example

The core of the exploit is summarized in this JavaScript snippet (reformatted for clarity):

// Create an invisible iframe for GitHub OAuth authorization
const iframe = document.createElement('iframe');
iframe.src = 'https://github.com/login/oauth/authorize?client_id=54fc42e15038794b7011&scope=public_repo&redirect_uri=https://www.webcomponents.org/element/ThomasOrlita/test2';
iframe.style.display = 'none';
document.body.appendChild(iframe);

// After some delay, capture the redirected URL to extract the OAuth code
setTimeout(() => {
  const url = new URL(iframe.contentWindow.location.href);
  const code = url.searchParams.get('code');

  // Target repository to star
  const repoToStar = 'kelseyhightower/nocode';

  // Use the code to star the repo via WebComponents.org API
  fetch(`/api/star/${repoToStar}?code=${code}`, { method: 'POST' });
}, 5000);

Implications and Impact

This vulnerability highlights how XSS can leverage OAuth flows to amplify the attack surface. By stealing the authorization code, attackers gain delegated access to GitHub accounts through WebComponents.org’s authentication process without needing the user’s GitHub password.

• Account Abuse: Attackers can star or fork repositories, potentially manipulating social proof and metrics on the user’s behalf.
• Trust Exploitation: The attack risks reputation damage as compromised accounts perform unauthorized actions.
• Broader Security Risks: Similar amplified attacks could target repositories with write or admin permissions if OAuth scopes were misconfigured.

Timeline and Remediation

Best Practices to Prevent OAuth-related XSS Exploits

Modern web applications must adopt multilayered defenses when integrating OAuth and similar authentication flows:

• Input Validation and Sanitization: Prevent injection of malicious URLs or JavaScript code via user inputs.
• Use Safe Redirect URIs: Enforce strict whitelist of OAuth redirect URIs to avoid attackers controlling redirection.
• Implement Content Security Policy (CSP): Restrict script execution and iframe sources to authorized domains.
• Adopt SameSite Cookies: Limit cross-site cookie sending to reduce session hijacking risk.
• Apply OAuth State Parameter: Use anti-CSRF tokens in OAuth requests to prevent authorization code interception.

Broader Context: OAuth Security Challenges in 2024

With growing adoption of OAuth for single sign-on and API access delegation, ensuring secure implementation is critical. According to a 2023 [OAuth Security Report by the OpenID Foundation](https://openid.net/2023-oauth-security-report/), misconfiguration and inadequate input sanitization remain top root causes of OAuth abuse incidents.

Additionally, security researchers highlight increasing risks of authorization code leakage through client-side vulnerabilities like XSS. Consequently, integrating strict security controls around OAuth endpoints, thorough penetration testing, and continuous monitoring are essential safeguards.

Conclusion

The stored XSS vulnerability on WebComponents.org serves as a case study underscoring the intricate relationship between web vulnerabilities and OAuth-based authentication risks. Attackers can leverage weaknesses in one vector to escalate privileges or manipulate authorized services.

Developers and security teams should prioritize robust input validation, enforce strict OAuth flow configurations, and adopt comprehensive security headers to minimize such risks. Keeping OAuth implementations up to date with best practices is vital in maintaining user trust and service integrity.

Additional Resources for Developers and Security Professionals

• OWASP Top Ten Web Application Security Risks – Overview of common vulnerabilities including XSS
• OAuth 2.0 Framework – Official OAuth documentation
• Google OAuth 2.0 Security Recommendations – Practical guidance on secure OAuth integration
• Content Security Policy (CSP) Guide – How to mitigate script injection attacks

Note: The reported vulnerability was responsibly disclosed and patched in 2018 to protect users and platform integrity.