Crosssubdomain

Crosssubdomain refers to interactions, data sharing, and policy enforcement across subdomains that belong to the same parent domain. It is common in web ecosystems where a site operates multiple subdomains—such as login.example.com and app.example.com—and needs to coordinate state or data without requiring the user to re-authenticate.

A primary mechanism for crosssubdomain coordination is cookie sharing. By setting a cookie with the Domain attribute to the parent domain (for example, Domain=.example.com), the cookie is sent to all subdomains under that domain. This enables unified sign-in, user preferences, and session management across subdomains. However, sharing cookies broadens the attack surface: cookies should be marked HttpOnly and Secure, use SameSite where appropriate, and sensitive session data should be protected with short lifetimes or alternative tokens.

Cross-subdomain communication often involves different origins, since subdomains are considered distinct origins under the same parent domain. When a web page on one subdomain needs data from another, mechanisms such as CORS for resource sharing or postMessage for controlled messaging are used. For embedded content, framing policies and content security measures help mitigate risks.

Security considerations are central to crosssubdomain patterns. Misconfigured cookies can lead to session hijacking, while cross-origin data exchange requires strict origin checks to prevent data leakage. Subdomain-level access should be minimized, and defenses against CSRF, subdomain takeover, and downgrade attacks should be maintained. Proper auditing of DNS, TLS certificates, and subdomain configurations is essential to reduce exposure.

Related concepts include single sign-on across subdomains, cross-origin resource sharing, and inter-subdomain messaging techniques used in modern web architectures.