CVE-2025-27794: Flarum Vulnerable to Session Hijacking via Authoritative Subdomain Cookie Overwrite

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren’t rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser’s security measures this does not seem to be exploitable as described.