How to Enable Single Sign-On (SSO) Across Your WordPress Multisite Network

One of the most common challenges for WordPress Multisite administrators is managing user logins across multiple domains or subdomains. Users expect a seamless experience where logging into one site grants them access to all sites they are authorized for within the network. This is often referred to as Single Sign-On (SSO). Based on community discussions, this guide will explain why this happens and walk you through the most effective solutions.

The Core Problem: Cookie Domains

By default, a standard WordPress Multisite installation is configured to handle a network of subdomains (e.g., site1.example.com, site2.example.com) or subdirectories (e.g., example.com/site1, example.com/site2) under a single primary domain. WordPress uses cookies to keep users logged in. These cookies are typically scoped to a specific domain.

When you introduce completely different top-level domains (TLDs) like domain-a.com and domain-b.com, the browser's security model prevents cookies from one domain from being read by another. This is why a user logged into site1.mydomain.com appears logged out when they navigate to site2.mydomain.com or, more drastically, a different TLD entirely.

Common Solutions for a Unified Login Experience

Solution 1: Configure Cookie Sharing in wp-config.php (For Subdomains)

If your network uses subdomains, you can often solve the login persistence issue by explicitly defining the cookie domain in your wp-config.php file. This tells WordPress to set its authentication cookies for the entire root domain, making them available to all subdomains.

define('COOKIE_DOMAIN', '.mydomain.com'); // Note the leading dot

Important Note: As seen in the community threads, this solution is primarily effective for subdomains of the same root domain. It will not work for mapping completely different TLDs (e.g., domain-a.com and domain-b.com). Some users have reported that setting this value to false can resolve login issues if other configurations cause problems.

Solution 2: Use a Single Sign-On (SSO) Plugin

For networks that use domain mapping with different TLDs, the most robust and commonly recommended solution is to implement a Single Sign-On plugin. These plugins work by centralizing the authentication process on one main site.

When a user attempts to access a restricted page on a mapped domain (e.g., domain-b.com), they are redirected to the main site to log in. After successful authentication, they are redirected back to the mapped domain with a token that proves their identity, effectively logging them in there as well.

The community frequently suggests exploring SSO plugins as a reliable method to achieve this cross-domain login behavior. It is advisable to search the WordPress Plugin Directory for "Multisite SSO" or "Single Sign-On" to find a solution that fits your specific network setup.

Solution 3: Centralized Login and Logout Pages

A related goal for many administrators is to force all logins and logouts to occur through a single, central URL (e.g., example.com/login). This improves user experience and simplifies management. This can often be achieved through a combination of:

• Custom code using filters like site_url and wp_redirect.
• Dedicated plugins designed to customize WordPress login behavior.
• Careful configuration of login links within your theme or menus.

A Note on User Access vs. Authentication

It is crucial to distinguish between being authenticated (logged in) and having access to a site's backend. A user may be logged into the entire network but still have no role or capabilities on a specific subsite. By default, a user registered on one subsite will have a "Subscriber" role on all other sites in the network, but they will not appear in the user list of a subsite until an administrator explicitly adds them and assigns a higher role. This allows for fine-grained control over who can actually administer or create content on each individual site.

Conclusion

Achieving a seamless login experience across a WordPress Multisite network, especially one with mapped domains, requires moving beyond the default configuration. For subdomain networks, tweaking the COOKIE_DOMAIN constant in wp-config.php is often sufficient. For more complex setups involving multiple top-level domains, implementing a Single Sign-On plugin is the most effective and widely recommended path forward by the community.