SSO & Cross-Site Sharing Security

TL;DR

Single Sign-On (SSO) makes things easier for users, but it can introduce security risks when sharing data between websites. This guide explains how to spot and fix potential problems with controlled cross-site sharing behind SSO.

Understanding the Risks

When you use SSO, a central identity provider (IdP) handles authentication. Websites (service providers or SPs) trust the IdP to verify users. This means if one website is compromised, others relying on that same SSO could be too. Cross-site sharing adds another layer of complexity because data flows between these trusted sites.

Solution Guide: Securing Controlled Cross-Site Sharing with SSO

1. Identify Your Data Flows: First, map out exactly what data is being shared between websites that use the same SSO. This includes:
   • What information is sent? (e.g., user profile details, session tokens)
   • Which websites are involved?
   • How is the data transferred? (e.g., API calls, redirects with parameters, embedded content)
2. Implement Strict Access Controls: Don’t assume SSO authentication automatically means safe sharing. Each website needs its own access controls.
   • Principle of Least Privilege: Users should only have access to the data they absolutely need on each site.
   • Role-Based Access Control (RBAC): Define roles with specific permissions, and assign users to those roles.
3. Secure API Communication: If websites share data via APIs:
   • HTTPS Only: Always use HTTPS to encrypt data in transit.
   • API Keys & Tokens: Use strong, unique API keys or tokens for authentication. Rotate these regularly.
   • Input Validation: Thoroughly validate all input received from other websites to prevent injection attacks (e.g., SQL injection, cross-site scripting).
4. CORS Configuration: Cross-Origin Resource Sharing (CORS) controls which domains can access resources on your website.
   • Be Specific: Don’t use wildcard (*) for allowed origins unless absolutely necessary. List the exact domains that are permitted to access your APIs.
   • Check Headers: Ensure your server is returning the correct CORS headers in its responses. For example:

     Access-Control-Allow-Origin: https://alloweddomain.com

5. Session Management Security: SSO often involves session cookies.
   • HttpOnly & Secure Flags: Set the HttpOnly flag to prevent JavaScript access and the Secure flag to ensure cookies are only sent over HTTPS.
   • SameSite Attribute: Use the SameSite attribute (Strict, Lax, or None) to control when cookies are sent with cross-site requests. Consider SameSite=Strict for sensitive data.

     Set-Cookie: sessionid=value; HttpOnly; Secure; SameSite=Strict

6. Content Security Policy (CSP): CSP helps prevent cross-site scripting attacks by controlling the resources a website can load.
   • Define Allowed Sources: Specify which domains are allowed to provide scripts, styles, images, etc.
   • Report Violations: Configure CSP to report violations so you can identify and address potential issues.

     Content-Security-Policy: default-src 'self'; script-src 'self' https://alloweddomain.com;

7. Regular Security Audits & Penetration Testing: Regularly assess your SSO configuration and cross-site sharing mechanisms for vulnerabilities.
   • Focus on Authentication Flows: Test the entire authentication process, including redirects and token handling.
   • Simulate Attacks: Attempt to exploit common web application vulnerabilities (e.g., XSS, CSRF).