How Can I Securely Share Cookies Across Different Domains?

Cross-Domain Cookie Complications

Cross-domain cookie sharing poses significant challenges due to security concerns and browser restrictions. Setting cookies across multiple domains is not straightforward, especially if not done through direct user interaction.

Addressing the Issue

Unfortunately, it's not possible for domain.com to set cookies for domain1.com directly. The only feasible solution involves leveraging the user's browser to connect with each domain separately, where each domain then sets its own cookie.

Verifying User Identity

Once cookies are set across domains, the next step is to verify user identity. This can be achieved through two main approaches:

• Back Channel: Establish direct communication between the sites to share login status information.
• GET/POST Token Passing: Pass a digitally signed token in HTTP requests that contains user identity and session status when redirecting users between domains.

Complexity and Recommendations

Implementing a cross-domain cookie solution is highly complex and requires specialized knowledge. It's advisable to adopt existing SSO frameworks rather than building your own. SimpleSAMLPHP is a recommended option for PHP implementations that provides a comprehensive solution for cross-domain user authentication and session management.

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