Weak Session IDs

After visiting your site, he opens up his browser debugger, and looks at the headers in the HTTP response. He notices that the Set-Cookie header includes a surprisingly small session ID.
A pensive hacker
Headers
  ▼ General
      Remote Address: 121.232.112.200:443
      Request Method: GET
      Status Code: 200 OK
  ▶ Request Headers
  ▼ Response Headers
      Set-Cookie: session_id=142983010