Channel authorization re-validated on subscribe, not only at initial handshake

A handshake-only authorization model is susceptible to time-of-check/time-of-use attacks: a user whose channel access is revoked after connecting can continue subscribing to restricted channels until the connection drops. CWE-285 (Improper Authorization) and OWASP A01 (Broken Access Control) both describe this failure — authorization decisions must be re-evaluated at the point of access, not cached from an earlier check. In a community app, this means a banned user or a user whose subscription lapses can still join private channels by issuing subscribe events on an existing connection.

Re-check channel permissions on every subscribe event, not only at connection time. Emit an error and skip socket.join() if the check fails.

socket.on('subscribe', async (channel: string) => {
  const allowed = await checkChannelAccess(socket.userId, channel);
  if (!allowed) {
    socket.emit('error', { code: 403, message: 'Forbidden', channel });
    return;
  }
  socket.join(channel);
  socket.emit('subscribed', { channel });
});

Never trust client-side authorization state. The server must be the sole authority on channel membership.

• ID: channel-auth-revalidation

• Severity: critical

• What to look for: Count all subscribe event handlers. For each, classify whether user permission is re-validated before emitting messages. Enumerate the authorization checks present per handler.

• Pass criteria: The subscribe handler validates that the user has permission to access the target channel before emitting any messages. 100% of subscribe handlers must re-validate permissions. Authorization must not rely solely on the initial handshake.

• Fail criteria: Channel access is only checked at initial handshake, or no per-channel authorization is implemented.

• Do NOT pass when: Authorization is checked only via client-side logic that the server trusts without verification — client-side-only auth is NOT a pass.

• Skip (N/A) when: Never — channel-level authorization is critical for multi-channel systems.

• Cross-reference: For authorization patterns and permission models, the Auth & Session Audit covers role-based access control.

• Detail on fail: "Subscribe handler does not re-validate channel permissions. User granted permission at handshake could subscribe to restricted channels later."

• Remediation: Validate channel access on every subscribe request:

  socket.on('subscribe', async (channel: string) => {
    const userId = socket.userId;
    const hasAccess = await checkChannelAccess(userId, channel);
    if (!hasAccess) {
      socket.emit('error', { message: 'Forbidden', channel });
      return;
    }
    socket.join(channel);
    socket.emit('subscribed', { channel });
  });