Flarum Vulnerable to Session Hijacking via Authoritative Subdomain Cookie Overwrite

Summary

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.

---

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

---

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., , not ).