Vulnerability in Google's OAuth Workflow Poses Security Risks

A critical vulnerability has been identified in Google's OAuth 2.0 authentication system, commonly used in the "Sign in with Google" feature. This flaw allows individuals to create unauthorized Google accounts associated with corporate email domains, potentially granting them access to various third-party applications like Slack and Zoom, even after they have left the organization.

Understanding the Vulnerability

The issue arises from Google's allowance for users to register Google accounts using any email address, not limited to Gmail. By exploiting email aliases and sub-addressing (e.g., using "user+alias@company.com"), individuals can create Google accounts that appear to belong to a corporate domain. These accounts are not visible to corporate administrators and can be used to access services that rely on Google OAuth for authentication.

This loophole enables former employees or malicious actors to maintain or gain unauthorized access to corporate resources. Since these shadow accounts are not managed by the organization's Google Workspace, they can persist undetected, posing significant security risks.

Implications for Organizations

The primary concern is that offboarding processes may fail to revoke access to critical applications. Even after an employee's official corporate account is deactivated, these unmanaged Google accounts can still be used to log in to services integrated with Google OAuth. This undermines security protocols and exposes sensitive data to potential breaches.

Mitigation Strategies

To address this vulnerability, organizations are advised to:

Disable "Sign in with Google": Where possible, disable Google OAuth authentication and enforce alternative methods such as Security Assertion Markup Language (SAML) for user authentication.
Implement Strict Access Controls: Use invite-only or LDAP group-based provisioning to ensure that only authorized users can access corporate applications.
Monitor for Unrecognized Accounts: Regularly audit user accounts and monitor for any unauthorized access, paying special attention to email aliases and sub-addresses.
Engage with Service Providers: Encourage third-party service providers to validate the hosted domain (HD) claim in OAuth tokens to ensure that only legitimate corporate accounts are granted access.

Conclusion

This vulnerability in Google's OAuth workflow highlights the need for organizations to re-evaluate their authentication mechanisms and offboarding procedures. By implementing stricter access controls and monitoring practices, companies can mitigate the risks associated with unauthorized access through unmanaged Google accounts.