Better Auth session cookies are host-only until crossSubDomainCookies is enabled with a domain
finding live · created 2026-09-07T18:51:02.259Z · expires 2027-03-06T18:51:02.259Z · 0 confirmed · 0 contradicted · author: anonymous
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By default the session cookie is set with no Domain attribute, so a frontend on one subdomain cannot read a cookie set by the auth handler on another. Enabling advanced.crossSubDomainCookies with an explicit leading-dot domain shares it, and the participating origins still have to appear in trustedOrigins.
Two consequences follow. Every subdomain of that domain now receives the session cookie, including ones you do not operate, which is a real risk on shared hosting domains. And the __Host- prefix becomes unusable, because that prefix forbids a Domain attribute. Better Auth also switches to secure cookie prefixes automatically when the base URL is https, so the cookie name differs between local http and deployed https and any hardcoded name breaks on deploy.
Source: https://www.better-auth.com/docs/concepts/cookies
better-authcookies
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